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_postgres.go

@@ -0,0 +1,77 @@
+//go:build test_db_postgres && !test_db_sqlite
+
+package paymentsdb
+
+import (
+	"database/sql"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/sqldb"
+	"github.com/stretchr/testify/require"
+)
+
+// NewTestDB is a helper function that creates a SQLStore backed by a SQL
+// database for testing.
+func NewTestDB(t testing.TB, opts ...OptionModifier) DB {
+	return NewTestDBWithFixture(t, nil, opts...)
+}
+
+// NewTestDBFixture creates a new sqldb.TestPgFixture for testing purposes.
+func NewTestDBFixture(t *testing.T) *sqldb.TestPgFixture {
+	pgFixture := sqldb.NewTestPgFixture(
+		t, sqldb.DefaultPostgresFixtureLifetime,
+	)
+	t.Cleanup(func() {
+		pgFixture.TearDown(t)
+	})
+	return pgFixture
+}
+
+// NewTestDBWithFixture is a helper function that creates a SQLStore backed by a
+// SQL database for testing.
+func NewTestDBWithFixture(t testing.TB,
+	pgFixture *sqldb.TestPgFixture, opts ...OptionModifier) DB {
+
+	var querier BatchedSQLQueries
+	if pgFixture == nil {
+		querier = newBatchQuerier(t)
+	} else {
+		querier = newBatchQuerierWithFixture(t, pgFixture)
+	}
+
+	store, err := NewSQLStore(
+		&SQLStoreConfig{
+			QueryCfg: sqldb.DefaultPostgresConfig(),
+		}, querier, opts...,
+	)
+	require.NoError(t, err)
+
+	return store
+}
+
+// newBatchQuerier creates a new BatchedSQLQueries instance for testing
+// using a PostgreSQL database fixture.
+func newBatchQuerier(t testing.TB) BatchedSQLQueries {
+	pgFixture := sqldb.NewTestPgFixture(
+		t, sqldb.DefaultPostgresFixtureLifetime,
+	)
+	t.Cleanup(func() {
+		pgFixture.TearDown(t)
+	})
+
+	return newBatchQuerierWithFixture(t, pgFixture)
+}
+
+// newBatchQuerierWithFixture creates a new BatchedSQLQueries instance for
+// testing using a PostgreSQL database fixture.
+func newBatchQuerierWithFixture(t testing.TB,
+	pgFixture *sqldb.TestPgFixture) BatchedSQLQueries {
+
+	db := sqldb.NewTestPostgresDB(t, pgFixture).BaseDB
+
+	return sqldb.NewTransactionExecutor(
+		db, func(tx *sql.Tx) SQLQueries {
+			return db.WithTx(tx)
+		},
+	)
+}