Fix WPT content-computed.html to not send actual image requests to example.com.

This WPT puts CSS like the following on a test element: content: url("https://www.example.com/picture.svg")

That is effecively equivalent to inserting an element like the following:

In particular, it triggers an actual web request to example.com, in an attempt to load the actual image. Firefox's test harness configuration detects this remote network-request and insta-crashes instead of executing it, which is why this test was flagged as crashing in Firefox. Really, the test needs to be fixed to avoid generating this image request entirely.

So, this patch adds display:none to the target element, which is sufficient to prevent the browser from loading images for that element. (This change doesn't impact the behavior that the test is actually trying to exercise, which is just parsing, not image loading and rendering.)

As part of this, I'm updating Firefox's in-tree test-failure annotations from a crash to a pile of subtest-failures, which are all due to https://bugzilla.mozilla.org/show_bug.cgi?id=1281158

Differential Revision: https://phabricator.services.mozilla.com/D198341

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1871766 gecko-commit: 1f5a522f7bb20848cc1dbe727fec97a7388e4679 gecko-reviewers: layout-reviewers, emilio