Bug 1780474 [wpt PR 34921] - Add a comment about the testing methodology, a=testonly

Bug 1780474 [wpt PR 34921] - Add a comment about the testing methodology, a=testonly

Automatic update from web-platform-tests
Add a comment about the testing methodology

This comes from a comment on [1] - more of an explanatory comment
is added to clarify why tests are skipped if they run too slowly.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/3760885

Bug: 1307772
Change-Id: I69b94218cf2b27eecbf2eac794bea9083e84c117
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3778249
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Mason Freed <masonf@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1026509}

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wpt-commits: deab937d37088151680a31dbe4871e474c535778
wpt-pr: 34921