Replace remote connections in test_http_stacktrace or skip locally

[boolean5]

We've noticed that test_http_stacktrace is flaky when running on CI (see here for a failure example).

This is probably because this test is still making a remote connection to https://gist.githack.com/gunesacar/b927d3fe69f3e7bf456da5192f74beea/raw/8d3e490b5988c633101ec45ef1443e61b1fd495e/inject_pixel.js

We could investigate whether we can replace remote connections in this test with local ones. If not, we should be skipping it locally as we do for test_browser_profile_coverage and test_name_resolution.

[vringar]

Maybe we can remove the remote connection by using the pref englehardt mentioned in #197