Firefox browser Fetch tests flaky

It is possible this is a general pthreads flakiness, and the fetch tests are just getting blamed because they are fairly early in the list of tests by name. However, they are not the first pthread test (e.g. `test_egl_with_proxy_to_pthread`), and I've not seen the ones before or after fail that I can remember, and the flake happens less than 1/2 the time (so the ones after should have a chance to fail too) so perhaps there is something more specific there.