Consitently switch to stretchr/testify

We were using stretchr/testify and test-go/testify inconsitently and
sometimes mixed in the same tests. This can cause very strange issue,
e.g when using things like mock.MatchedBy().

This moves all our code to stretchr/testify, which seems to be far
more active and maintained then test-go/testify.

services/invitations/pkg/backends/keycloak/backend_test.go

@@ -13,7 +13,7 @@ import (
 	"github.com/owncloud/ocis/v2/services/invitations/pkg/invitations"
 	"github.com/owncloud/ocis/v2/services/invitations/pkg/mocks"
 	"github.com/stretchr/testify/assert"
-	"github.com/test-go/testify/mock"
+	"github.com/stretchr/testify/mock"
 )
 
 const (