coldcard: extract transaction signing helpers

### hwilib/devices/coldcard.py
@@ -155,6 +155,81 @@ def get_master_fingerprint(self) -> bytes:
         # quick method to get fingerprint of wallet
         return struct.pack('<I', self.device.master_fingerprint)
 
+    def _sign_tx_once(self, psbt: PSBT) -> PSBT:
+        # Get psbt in hex and then make binary
+        fd = io.BytesIO(base64.b64decode(psbt.serialize()))
+
+        # learn size (portable way)
+        sz = fd.seek(0, 2)
+        fd.seek(0)
+
+        left = sz
+        chk = sha256()
+        for pos in range(0, sz, MAX_BLK_LEN):
+            here = fd.read(min(MAX_BLK_LEN, left))
+            if not here:
+                break
+            left -= len(here)
+            result = self.device.send_recv(CCProtocolPacker.upload(pos, sz, here))
+            assert result == pos
+            chk.update(here)
+
+        # do a verify
+        expect = chk.digest()
+        result = self.device.send_recv(CCProtocolPacker.sha256())
+        assert len(result) == 32
+        if result != expect:
+            raise DeviceFailureError("Wrong checksum:\nexpect: %s\n   got: %s" % (b2a_hex(expect).decode('ascii'), b2a_hex(result).decode('ascii')))
+
+        # start the signing process
+        ok = self.device.send_recv(CCProtocolPacker.sign_transaction(sz, expect), timeout=None)
+        assert ok is None
+        if self.device.is_simulator:
+            self.device.send_recv(CCProtocolPacker.sim_keypress(b'y'))
+
+        print("Waiting for OK on the Coldcard...", file=sys.stderr)
+
+        while 1:
+            time.sleep(0.250)
+            done = self.device.send_recv(CCProtocolPacker.get_signed_txn(), timeout=None)
+            if done is None:
+                continue
+            break
+
+        if len(done) != 2:
+            raise DeviceFailureError('Failed: %r' % done)
+
+        result_len, result_sha = done
+
+        result = self.device.download_file(result_len, result_sha, file_number=1)
+
+        psbt = PSBT()
+        psbt.deserialize(base64.b64encode(result).decode())
+        return psbt
+
+    def _sign_without_policy_names(
+        self,
+        psbt: PSBT,
+        master_fp: bytes,
+    ) -> PSBT:
+        # For multisigs, we may need to do multiple passes if we appear in an
+        # input multiple times.
+        passes = 1
+        if not self.is_edge:
+            for psbt_in in psbt.inputs:
+                our_keys = 0
+                for key in psbt_in.hd_keypaths.keys():
+                    keypath = psbt_in.hd_keypaths[key]
+                    if keypath.fingerprint == master_fp and key not in psbt_in.partial_sigs:
+                        our_keys += 1
+                if our_keys > passes:
+                    passes = our_keys
+
+        for _ in range(passes):
+            psbt = self._sign_tx_once(psbt)
+
+        return psbt
+
     @coldcard_exception
     def sign_tx(
         self,
@@ -176,73 +251,10 @@ def sign_tx(
         xpub = self.device.send_recv(CCProtocolPacker.get_xpub('m/0\''), timeout=None)
         master_fp = get_xpub_fingerprint(xpub)
 
-        # For multisigs, we may need to do multiple passes if we appear in an input multiple times
-        passes = 1
-        if not self.is_edge:
-            for psbt_in in psbt.inputs:
-                our_keys = 0
-                for key in psbt_in.hd_keypaths.keys():
-                    keypath = psbt_in.hd_keypaths[key]
-                    if keypath.fingerprint == master_fp and key not in psbt_in.partial_sigs:
-                        our_keys += 1
-                if our_keys > passes:
-                    passes = our_keys
-
         if psbt.version == 2 and not self._supports_psbt_v2():
             psbt.convert_to_v0()
 
-        for _ in range(passes):
-            # Get psbt in hex and then make binary
-            fd = io.BytesIO(base64.b64decode(psbt.serialize()))
-
-            # learn size (portable way)
-            sz = fd.seek(0, 2)
-            fd.seek(0)
-
-            left = sz
-            chk = sha256()
-            for pos in range(0, sz, MAX_BLK_LEN):
-                here = fd.read(min(MAX_BLK_LEN, left))
-                if not here:
-                    break
-                left -= len(here)
-                result = self.device.send_recv(CCProtocolPacker.upload(pos, sz, here))
-                assert result == pos
-                chk.update(here)
-
-            # do a verify
-            expect = chk.digest()
-            result = self.device.send_recv(CCProtocolPacker.sha256())
-            assert len(result) == 32
-            if result != expect:
-                raise DeviceFailureError("Wrong checksum:\nexpect: %s\n   got: %s" % (b2a_hex(expect).decode('ascii'), b2a_hex(result).decode('ascii')))
-
-            # start the signing process
-            ok = self.device.send_recv(CCProtocolPacker.sign_transaction(sz, expect), timeout=None)
-            assert ok is None
-            if self.device.is_simulator:
-                self.device.send_recv(CCProtocolPacker.sim_keypress(b'y'))
-
-            print("Waiting for OK on the Coldcard...", file=sys.stderr)
-
-            while 1:
-                time.sleep(0.250)
-                done = self.device.send_recv(CCProtocolPacker.get_signed_txn(), timeout=None)
-                if done is None:
-                    continue
-                break
-
-            if len(done) != 2:
-                raise DeviceFailureError('Failed: %r' % done)
-
-            result_len, result_sha = done
-
-            result = self.device.download_file(result_len, result_sha, file_number=1)
-
-            psbt = PSBT()
-            psbt.deserialize(base64.b64encode(result).decode())
-
-        return psbt
+        return self._sign_without_policy_names(psbt, master_fp)
 
     @coldcard_exception
     def sign_message(self, message: Union[str, bytes], keypath: str) -> str: