Don't fail tests for known flaky test results.

[flackr]

There is at least one test which usually runs and fails but sometimes doesn't run:

FAIL /scroll-animations/view-timelines/svg-graphics-element-002.html View timeline attached to SVG graphics element

This leads to semi-regular PR verification failures. We should add some way to skip or ignore or allow for various test results. I'm thinking the simplest might be to add an ignore list which would be used to filter the test expectations of these result lines before diffing.