Show spec links from tests and allow searching by them

[foolip]

Moved from web-platform-tests/results-collection#29.

The title is the use case. An alternative implementation would be to allow searching by all test content and have that search be powerful enough to support the use case. This would be quite handy in order to find tests containing certain other strings too.

The "filter by spec link" use case is one which the CSS WG would probably make use of.

[foolip]

@mdittmer FYI this is in part a search feature request.

There's also some overlap with #372, which is about spec links at a higher level.

[lukebjerring]

This feature is a possible atom/extension of the searchcache.

[foolip]

w3c/csswg-drafts#4303 is a use case from the wild, to filter by spec level. It assumes some uniformity of the links, but that's pretty easy to bring about within a single directory.

[lukebjerring]

Ability to search by spec URL is covered by #1489

[lukebjerring]

And the UI-surfacing part is covered by #372