paymentsdb: add harness to run payment db agnostic tests

In commit adds the harness which will be used to run db agnostic
tests against the kv and sql backend. We have adopted all the
unit tests so far so that with this commit all the payment tests
not specifically put into the kv_store_test.go should all pass
for all backends.

payments/db/test_kvdb.go

@@ -1,3 +1,5 @@
+//go:build !test_db_sqlite && !test_db_postgres
+
 package paymentsdb
 
 import (