Add two methods to the axios_utils Jest mock:
- `waitFor(url)`, which returns a Promise that resolves when the
next request to `url` finishes.
- `waitForAll()`, which returns a Promise that resolves when all
pending requests finish.
Moved the block that fails tests on unmocked axios requests from
test_setup to its own mock, and added a jQuery mock that fails
tests if they use unmocked $.ajax().
