Tidy: allow feature-gate tests to be ui tests

Tidy: allow feature-gate tests to be ui tests

ui tests are the future, especially since the
recent improvement where we have gained
checking and requiring of //~ERROR comments.

The tidy feature-gate test check is intended
to be 50% an actual insurance that there is
a check, and 50% to be a teacher that such
checks are required.

With this commit applied, newbies might
interpret stuff wrongly and create tests
that don't fail but succeed instead.

This is not what feature gate tests are
for though. Therefore, in a later step,
when only ui tests are allowed to be feature
gate tests, we will add checking to ensure
that a file marked as gate test is actually
required to be a compilation failure.

Right now implementing such a check is a
bit annoying as one needs to only do it
when the compile-fail test is in the
ui test suite :/.