Disable test external/wpt/event-timing/mouseleave.html

This test is flaky on all builds and we're seeing the same flakiness on
Edge, so adding test to TestExpectations

See test history: https://ci.chromium.org/ui/test/chromium/ninja%3A%2F%2F%3Ablink_wpt_tests%2Fexternal%2Fwpt%2Fevent-timing%2Fmouseleave.html

Bug: 40279192
Change-Id: Ifed09a61709cabae97d23f8df7438db0c40f6561
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5552021
Reviewed-by: Anupam Snigdha <snianu@microsoft.com>
Commit-Queue: Stephanie Zhang <stephanie.zhang@microsoft.com>
Reviewed-by: Sanket Joshi <sajos@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#1309426}