Add helper to wait for axios requests in frontend tests

What does this MR do?

Fixes #60972 (closed)

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15896

Adds two methods to the axios_utils object:

• waitFor(url, callback) that waits for a request to the given URL and runs a callback after it's done.
• waitForAll(callback) that waits for all requests to finish and then runs a callback.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
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• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers