Fix various random CI image test failures

[kkoreilly]

Describe the bug

Almost everything is working with #1347, but there are a few tests that randomly fail, marked with TODO(#1456).

For tests like TestSnackbarTime, we probably just want to change the times so they are farther apart and there is less chance of failure.

How to reproduce

Remove the t.Skips on the tests with the aforementioned TODO and then run CI.

Example code

Relevant output

Platform

macOS

[kkoreilly]

I had to temporarily increase the imagex.Assert tolerance to 5 for this; once #1457 is done, if this issue is still not resolved, we should debug further.