w3c-test.org link for PR results is broken/confusing

[zcorpan]

See e.g.
https://wpt.fyi/results/css/css-ui/control-internal-pseudo-elements-getComputedStyle-001.html?sha=c12c73e623

There's a w3c-test.org link, but the test is not in master, it results in a 404, and this is confusing:

w3c/csswg-drafts#3980 (comment)

Either remove the link or make it point to the right submissions/ URL if there is one.

[lukebjerring]

This is quite an edge-case; the only users that will wind up looking at a PR that's adding a test are the author or reviewer of the PR, who have the context on the test being newly added (and not existing in master).

However, when this does get tackled, the fix is to omit the link when the fetched manifest doesn't have an entry for the test that we're currently looking at (which is a small branch away from the code that already maps generated tests to their source file, etc).