The test was getting stuck because it was expecting a favicon.ico
request that never happened. The test logic was copied from
test_ext_webrequest_filter.html which suffers from exactly the same
issue. Since the favicon.ico test was meant to serve as a regression
test, this patch fixes only one of the two tests.

This patch also changes "http" to "https" in file_simple_xhr.html to get
the test to run successfully with the http3 test configuration, as
explained at https://bugzilla.mozilla.org/show_bug.cgi?id=1482983#c71

Differential Revision: https://phabricator.services.mozilla.com/D174505