paymentsdb: declare helper functions and add comments

diff --git a/payments/db/kv_store_test.go b/payments/db/kv_store_test.go
index b3d7887..caa46b5 100644
--- a/payments/db/kv_store_test.go
+++ b/payments/db/kv_store_test.go
@@ -20,6 +20,8 @@ import (
 
 // TestKVStoreDeleteNonInFlight checks that calling DeletePayments only
 // deletes payments from the database that are not in-flight.
+//
+// TODO(ziggie): Make this test db agnostic.
 func TestKVStoreDeleteNonInFlight(t *testing.T) {
 	t.Parallel()
 
@@ -249,9 +251,11 @@ type htlcStatus struct {
 
 // fetchPaymentIndexEntry gets the payment hash for the sequence number provided
 // from our payment indexes bucket.
-func fetchPaymentIndexEntry(_ *testing.T, p *KVStore,
+func fetchPaymentIndexEntry(t *testing.T, p *KVStore,
 	sequenceNumber uint64) (*lntypes.Hash, error) {
 
+	t.Helper()
+
 	var hash lntypes.Hash
 
 	if err := kvdb.View(p.db, func(tx walletdb.ReadTx) error {
@@ -319,6 +323,8 @@ func assertNoIndex(t *testing.T, p DB, seqNr uint64) {
 func makeFakeInfo(t *testing.T) (*PaymentCreationInfo,
 	*HTLCAttemptInfo) {
 
+	t.Helper()
+
 	var preimg lntypes.Preimage
 	copy(preimg[:], rev[:])
 
@@ -677,6 +683,8 @@ func putDuplicatePayment(t *testing.T, duplicateBucket kvdb.RwBucket,
 
 // TestQueryPayments tests retrieval of payments with forwards and reversed
 // queries.
+//
+// TODO(ziggie): Make this test db agnostic.
 func TestQueryPayments(t *testing.T) {
 	// Define table driven test for QueryPayments.
 	// Test payments have sequence indices [1, 3, 4, 5, 6, 7].
diff --git a/payments/db/payment_test.go b/payments/db/payment_test.go
index ea5fcb8..fcd82af 100644
--- a/payments/db/payment_test.go
+++ b/payments/db/payment_test.go
@@ -112,6 +112,8 @@ type payment struct {
 // the payments slice. Each payment will receive one failed HTLC and another
 // HTLC depending on the final status of the payment provided.
 func createTestPayments(t *testing.T, p DB, payments []*payment) {
+	t.Helper()
+
 	attemptID := uint64(0)
 
 	for i := 0; i < len(payments); i++ {
@@ -193,7 +195,9 @@ func createTestPayments(t *testing.T, p DB, payments []*payment) {
 
 // assertRouteEquals compares to routes for equality and returns an error if
 // they are not equal.
-func assertRouteEqual(a, b *route.Route) error {
+func assertRouteEqual(t *testing.T, a, b *route.Route) error {
+	t.Helper()
+
 	if !reflect.DeepEqual(a, b) {
 		return fmt.Errorf("HTLCAttemptInfos don't match: %v vs %v",
 			spew.Sdump(a), spew.Sdump(b))
@@ -239,7 +243,7 @@ func assertPaymentInfo(t *testing.T, p DB, hash lntypes.Hash,
 	}
 
 	htlc := payment.HTLCs[a.AttemptID]
-	if err := assertRouteEqual(&htlc.Route, &a.Route); err != nil {
+	if err := assertRouteEqual(t, &htlc.Route, &a.Route); err != nil {
 		t.Fatal("routes do not match")
 	}
 
@@ -331,7 +335,9 @@ func assertDBPayments(t *testing.T, paymentDB DB, payments []*payment) {
 }
 
 // genPreimage generates a random preimage.
-func genPreimage() ([32]byte, error) {
+func genPreimage(t *testing.T) ([32]byte, error) {
+	t.Helper()
+
 	var preimage [32]byte
 	if _, err := io.ReadFull(rand.Reader, preimage[:]); err != nil {
 		return preimage, err
@@ -344,7 +350,7 @@ func genPreimage() ([32]byte, error) {
 func genInfo(t *testing.T) (*PaymentCreationInfo, *HTLCAttemptInfo,
 	lntypes.Preimage, error) {
 
-	preimage, err := genPreimage()
+	preimage, err := genPreimage(t)
 	if err != nil {
 		return nil, nil, preimage, fmt.Errorf("unable to "+
 			"generate preimage: %v", err)
@@ -1466,7 +1472,7 @@ func TestSwitchFail(t *testing.T) {
 			len(payment.HTLCs))
 	}
 
-	err = assertRouteEqual(&payment.HTLCs[0].Route, &attempt.Route)
+	err = assertRouteEqual(t, &payment.HTLCs[0].Route, &attempt.Route)
 	if err != nil {
 		t.Fatalf("unexpected route returned: %v vs %v: %v",
 			spew.Sdump(attempt.Route),