Move about:srcdoc WPT tests to external.
There is nothing about these tests that requires them to be internal,
so moving them to an external directory.
Bug: 40165505
Change-Id: I3fe343b35c147dccdb1af46e8057afaaffa4182e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5799985
Commit-Queue: Dominic Farolino dom@chromium.org
Reviewed-by: Dominic Farolino dom@chromium.org
Cr-Commit-Position: refs/heads/main@{#1344620}