Bug 1932978 [wpt PR 49339] - Update READMEs for animation tests, a=testonly

Automatic update from web-platform-tests
Update READMEs for animation tests

First in a series of patches to document best practices for animation
tests. This patch adds a readme for non-WPT animation tests, and
updates the readme for CSS transitions.  Each tip was recently used to
fix a flake / failure introduced with the migration of css transition
tests from using run_web_test.py to run_wpt_test.py.

Change-Id: Iaba7ea397c4f55a090b08a71c046b96c64453bd9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6037955
Reviewed-by: Olga Gerchikov <gerchikomicrosoft.com>
Commit-Queue: Kevin Ellis <keverschromium.org>
Cr-Commit-Position: refs/heads/main{#1387067}

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wpt-commits: c27cc4faeeb1b4b5ce7f98e29a7f4aba5d6ac6ac
wpt-pr: 49339

UltraBlame original commit: 1d3bbbcde9e521bf0e9081d929ab9ba0e05c803b

Author: marco-c