graph/db: expand TestPopulateViaMigration for easy testing

diff --git a/graph/db/benchmark_test.go b/graph/db/benchmark_test.go
index bdb7954..3d690e3 100644
--- a/graph/db/benchmark_test.go
+++ b/graph/db/benchmark_test.go
@@ -240,6 +240,11 @@ func connectKVDBSqlite(t testing.TB, dbPath, fileName string) V1Store {
 
 // connectBBoltDB creates a new BBolt database connection for testing.
 func connectBBoltDB(t testing.TB, dbPath, fileName string) V1Store {
+	return newKVStore(t, kvdbBBolt(t, dbPath, fileName))
+}
+
+// kvdbBBolt creates a new bbolt backend for testing.
+func kvdbBBolt(t testing.TB, dbPath, fileName string) kvdb.Backend {
 	cfg := &kvdb.BoltBackendConfig{
 		DBPath:            dbPath,
 		DBFileName:        fileName,
@@ -252,7 +257,7 @@ func connectBBoltDB(t testing.TB, dbPath, fileName string) V1Store {
 	kvStore, err := kvdb.GetBoltBackend(cfg)
 	require.NoError(t, err)
 
-	return newKVStore(t, kvStore)
+	return kvStore
 }
 
 // newKVStore creates a new KVStore instance for testing using a provided
@@ -425,45 +430,122 @@ func TestPopulateDBs(t *testing.T) {
 }
 
 // TestPopulateViaMigration is a helper test that can be used to populate a
-// local native SQL graph from a kvdb-sql graph using the migration logic.
+// local native SQL graph from a kvdbgraph using the migration logic.
 //
 // NOTE: the testPostgres variable can be set to true to test with a
 // postgres backend instead of the kvdb-sqlite backend.
 //
 // NOTE: this is a helper test and is not run by default.
 func TestPopulateViaMigration(t *testing.T) {
+	// ======= STEP 0 ===========
+	// Comment out this SKipf line.
 	t.Skipf("Skipping local helper test")
 
-	// Set this to true if you want to test with a postgres backend.
-	// By default, we use a kvdb-sqlite backend.
-	testPostgres := false
+	const (
+		srcBBolt    = "kvdb-bbolt"
+		srcSQLite   = "kvdb-sqlite"
+		srcPostgres = "kvdb-postgres"
+	)
 
-	ctx := context.Background()
+	// ======= STEP 1 ===========
+	// Set your chosen SOURCE type by uncommenting the corresponding line
+	// below. By default, a kvdb-sqlite source is chosen.
+	srcDB := srcSQLite
+	// srcDB := srcBBolt
+	// srcDB := srcPostgres
+
+	// ======= STEP 2 ============
+	// Set this variable to the correct genesis hash of the source
+	// DB. By default, mainnet is assumed.
+	chain := *chaincfg.MainNetParams.GenesisHash
+
+	// ======= STEP 3 (ignore if source is postgres) ==============
+	// If your source destination is bbolt or sqlite, then set this to the
+	// path where your source database can be found.
+	const sourceDBPath = "testdata"
+
+	// ======= STEP 4 (only if source is bbolt!) ============
+	// If your source destination is bbolt, then set this to the name of
+	// the bbolt file that contains the channel graph data.
+	const sourceBBoltName = "channel.db"
+
+	// ======= STEP 5 (only if source is sqlite!) ============
+	// If your source destination is sqlite, then set this to the name of
+	// the sqlite file that contains the channel graph data.
+	const sourceSQLiteName = "channel.sqlite"
+
+	// ======= STEP 6 (only if source is postgres!) ============
+	// Set the DNS of your kvdb postgres instance below. This should be the
+	// same as what you have set in the config of the LND node that
+	// populated the instance (ie, whatever your --db.postgres.dsn is set
+	// to).
+	const kvdbPostgresDNS = "postgres://user@host/db_name"
+
+	// ======== STEP 7 ========================
+	// Finally, pick your destination DB! You can choose either SQLite or
+	// Postgres.
+	testSQLite := true
+
+	// ======== STEP 8 (only if destination is sqlite) ========
+	// Set the path where you want to create the destination SQLite
+	// database. This should be a directory that exists and is writable.
+	const destSQLitePath = "testdata"
+
+	// ======== STEP 9 (only if destination is sqlite) ========
+	// Pick a name for your destination SQLite database file.
+	// NOTE: if you run this test again, delete the previously created
+	// file first.
+	const destSQLiteFile = "lnd-graph-test.sqlite"
+
+	// ======== STEP 10 (only if destination is postgres) ========
+	// NB: this has some additional steps:
+	// 1. First, connect to your destination postgres instance: example:
+	//	 $ psql -U ellemouton -d postgres
+	// 2. Now, create the test database:
+	//	CREATE DATABASE graphtest;
+	// NOTE: if you restart this test for postgres, it helps to first drop
+	// the new database & recreate it.
+	// NOTE: the database name that you use above must be whatever you will
+	// use in the DNS you set below.
+	const sqlPostgresDNS = "postgres://user@host/graphtest"
+
+	// ======= YOUR WORK IS DONE =============
+
+	// Connect to source database.
+	var srcKVDB kvdb.Backend
+	switch srcDB {
+	case srcBBolt:
+		srcKVDB = kvdbBBolt(t, sourceDBPath, sourceBBoltName)
+	case srcSQLite:
+		srcKVDB = kvdbSqlite(t, sourceDBPath, sourceSQLiteName)
+	case srcPostgres:
+		srcKVDB = kvdbPostgres(t, kvdbPostgresDNS)
+	default:
+		t.Fatalf("Unsupported source database backend: %s", srcDB)
+	}
+
+	// Connect to destination database.
+	cfg := sqldb.DefaultSQLiteConfig()
+	dstSQL := sqlSQLite(t, destSQLitePath, destSQLiteFile)
+	if !testSQLite {
+		cfg = sqldb.DefaultPostgresConfig()
+		dstSQL = sqlPostgres(t, sqlPostgresDNS)
+	}
 
 	// Set up a logger so we can see the migration progress.
 	logger := btclog.NewDefaultHandler(os.Stdout)
 	UseLogger(btclog.NewSLogger(logger))
 	log.SetLevel(btclog.LevelDebug)
 
-	var (
-		cfg     = sqldb.DefaultSQLiteConfig()
-		srcKVDB = kvdbSqlite(t, kvdbSqlitePath, kvdbSqliteFile)
-		dstSQL  = sqlSQLite(t, nativeSQLSqlitePath, nativeSQLSqliteFile)
-	)
-	if testPostgres {
-		cfg = sqldb.DefaultPostgresConfig()
-		srcKVDB = kvdbPostgres(t, kvdbPostgresDNS)
-		dstSQL = sqlPostgres(t, nativeSQLPostgresDNS)
-	}
-
 	// Use the graph migration to populate the SQL graph from the
 	// kvdb graph.
+	ctx := context.Background()
 	err := dstSQL.ExecTx(
 		ctx, sqldb.WriteTxOpt(), func(queries SQLQueries) error {
 			return MigrateGraphToSQL(
 				ctx, &SQLStoreConfig{
 					QueryCfg:  cfg,
-					ChainHash: dbTestChain,
+					ChainHash: chain,
 				}, srcKVDB, queries,
 			)
 		}, func() {},