feat: add embedded testing server for postgres (#9955)

# Which Problems Are Solved

1. there was no embedded database to run tests against
2. there were no tests for postgres/migrate
3. there was no test setup for repository which starts a client for the
embedded database

# How the Problems Are Solved

1. postgres/embedded package was added
2. tests were added
3. TestMain was added incl. an example test

# Additional Changes

none

# Additional Context

closes #9934

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/v3/storage/database/repository/org_test.go

@@ -0,0 +1,16 @@
+package repository
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestBla is an example and can be removed later
+func TestBla(t *testing.T) {
+	var count int
+	err := pool.QueryRow(context.Background(), "select count(*) from zitadel.instances").Scan(&count)
+	assert.NoError(t, err)
+	assert.Equal(t, 0, count)
+}

backend/v3/storage/database/repository/repository_test.go

@@ -0,0 +1,41 @@
+package repository
+
+import (
+	"context"
+	"log"
+	"os"
+	"testing"
+
+	"github.com/zitadel/zitadel/backend/v3/storage/database"
+	"github.com/zitadel/zitadel/backend/v3/storage/database/dialect/postgres/embedded"
+)
+
+func TestMain(m *testing.M) {
+	os.Exit(runTests(m))
+}
+
+var pool database.Pool
+
+func runTests(m *testing.M) int {
+	connector, stop, err := embedded.StartEmbedded()
+	if err != nil {
+		log.Fatalf("unable to start embedded postgres: %v", err)
+	}
+	defer stop()
+
+	ctx := context.Background()
+
+	pool, err = connector.Connect(ctx)
+	if err != nil {
+		log.Printf("unable to connect to embedded postgres: %v", err)
+		return 1
+	}
+
+	err = pool.Migrate(ctx)
+	if err != nil {
+		log.Printf("unable to migrate database: %v", err)
+		return 1
+	}
+
+	return m.Run()
+}