Switch to Karma 6

[Dumluregn]

Are you reporting a feature request or a bug?

Task

Details

There are currently two issues:

• Karma seem to start a second instance due to some Chrome error;
• IE11 tests are failing (not only those mentioned in #74).

We should take care of both.

[Dumluregn]

We actually did this in https://github.com/ckeditor/ckeditor4-angular/pull/171 🤔