Wait for Karma tests that have pending requests left

What does this MR do?

Waits after each test Karma test run as long as there are pending requests left. This ensures that pending requests do not interfere with the following tests. Also this way logged errors from requests can be matched more easily with the test causing them.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/50664

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides