Text Fragments: Fixed test to open link in new tab to work on MacOS and moved it into mozilla-specific tests.
Apparently, synthesizing a CTRL/CMD-Click does not work reliably in Chrome/Safari, therefore the test is moved to mozilla-only web-platform tests.
Differential Revision: https://phabricator.services.mozilla.com/D219724
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1913935
gecko-commit: aaaf48d51fe5fd56e8e05ed25ffa46fb306352cb
gecko-reviewers: dom-core, masayuki