Auto merge of #18463 - mrobinson:disable-position-sticky-tests, r=jdm

Disable position:sticky tests

Sending scroll positions to Webender is way too flaky to run the
position:sticky tests at this time. We need to disable them for now, so
that other Servo work can continue.

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tests/wpt/mozilla/meta/css/css-position-3/__dir__.ini

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+disabled: Sending multiple scrolling positions to WebRender is very flaky in the test harness