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/dialect/postgres/migration/migrationgs_test.go

@@ -0,0 +1,60 @@
+package migration_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/muhlemmer/gu"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/zitadel/zitadel/backend/v3/storage/database"
+	"github.com/zitadel/zitadel/backend/v3/storage/database/dialect/postgres/embedded"
+)
+
+func TestMigrate(t *testing.T) {
+	tests := []struct {
+		name string
+		stmt string
+		args []any
+		res  []any
+	}{
+		{
+			name: "schema",
+			stmt: "SELECT EXISTS(SELECT 1 FROM information_schema.schemata where schema_name = 'zitadel') ;",
+			res:  []any{true},
+		},
+		{
+			name: "001",
+			stmt: "SELECT EXISTS(SELECT 1 FROM pg_catalog.pg_tables WHERE schemaname = 'zitadel' and tablename=$1)",
+			args: []any{"instances"},
+			res:  []any{true},
+		},
+	}
+
+	ctx := context.Background()
+
+	connector, stop, err := embedded.StartEmbedded()
+	require.NoError(t, err, "failed to start embedded postgres")
+	defer stop()
+
+	client, err := connector.Connect(ctx)
+	require.NoError(t, err, "failed to connect to embedded postgres")
+
+	err = client.(database.Migrator).Migrate(ctx)
+	require.NoError(t, err, "failed to execute migration steps")
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := make([]any, len(tt.res))
+			for i := range got {
+				got[i] = new(any)
+				tt.res[i] = gu.Ptr(tt.res[i])
+			}
+
+			require.NoError(t, client.QueryRow(ctx, tt.stmt, tt.args...).Scan(got...), "failed to execute check query")
+
+			assert.Equal(t, tt.res, got, "query result does not match")
+		})
+	}
+}