Fix transient failure in add_list_delete_branches_spec.rb

What does this MR do?

Sometimes add_list_delete_branches_spec.rb would not wait after deleting a branch before checking that the branch was done. It would fail when the UI was still processing.

This checks that the loading icon is gone before returning from delete_branch.

It also makes sure credentials are added properly after recent changes to QA::Git::Repository

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/quality/nightly/issues/62

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer