Auto merge of #29664 - mrobinson:fix-animation-flakiness, r=mukilan

Fix flakiness in animation tests

1. When updating the animation timeline, ensure that nodes that are
    animating are marked dirty, if necessary, so any style queries will
    force a layout flush.
2. Disable the problematic transition test suites, as they are in Gecko.
    These suites often fail when Servo is so overloaded that it cannot
    deliver frames fast enough to get more than two samples during the
    animation lifecycle.

Fixes #28334.
Fixes #26435.
Fixes #21486.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #28334, #26435, #21486.

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- [x] There are tests for these changes

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