When debugging the test failures in this test, I noticed that the info() messages indicated we *were* using
moz-extension icon references even when we shouldn't be - they just didn't include the 'fox.svg' bit.
When pausing in the debugger, you can see that all the buttons are blank - we don't load any icon in this case.
This seemed bad, so I updated the test to actually check if we're using a moz-extension URI at all,
and then updated the implementation to actually make it work.
MozReview-Commit-ID: GGXaivJrzxj