channeldb: mark ApplyMigration as a test helper

With this change, errors from migrations will have the proper local line
number.

channeldb/migtest/migtest.go

@@ -41,6 +41,8 @@ func ApplyMigration(t *testing.T,
 	beforeMigration, afterMigration, migrationFunc func(tx kvdb.RwTx) error,
 	shouldFail bool) {
 
+	t.Helper()
+
 	cdb, cleanUp, err := MakeDB()
 	defer cleanUp()
 	if err != nil {
@@ -55,6 +57,8 @@ func ApplyMigration(t *testing.T,
 	}
 
 	defer func() {
+		t.Helper()
+
 		if r := recover(); r != nil {
 			err = newError(r)
 		}