Fix image-decode-path-changes.html to not hit the image cache.
With my patches for bug 1076583, this test would start failing the third
subtest, due to the interaction described in 1.
If the svg load is synchronous, per spec both promises resolve, but that's not
what the test expects. Chromium behaves like us (with the patches) in the
reduced test-case from the spec issue, which is a bit confusing.
Differential Revision: https://phabricator.services.mozilla.com/D218059
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1910599
gecko-commit: 99ec29dc8ddcfd17cd8ece0ef065302bbe6890c3
gecko-reviewers: smaug