Add a test for setting .src then .loading = "lazy", ensuring load is not sync.
src-then-lazy-load.html passes in Chromium which means that the initial
diagnostic wasn't quite accurate. (you can test that by adding a dummy
random query string somewhere in the original test-case from the blocked
bug).
I still think the test is still probably worth landing.
Add a test for out of band loads somehow triggering lazy loads, see
linked spec issue.
Differential Revision: https://phabricator.services.mozilla.com/D226706
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1926666
gecko-commit: bc523d3640536476fa895d1e598a3c5c0c9bf614
gecko-reviewers: jrmuizel