[web] Allow tests to run with multiple renderers

[harryterkelsen]

This change creates the 'ui' test subdirectory, which is for tests using pure dart:ui API which we want to test with all renderers.

Pre-launch Checklist

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[mdebbar]

LGTM!

[harryterkelsen]

I noticed that loading color_test.dart appears a LOT in the outputs

[harryterkelsen]

So do several tests, actually. And when I run locally I don't see loading color_test.dart too many times.