tests: introduce igt_require

tests: introduce igt_require

Since igt_skip has funny control flow we can abuse it and make it work
like a special kind of assert which automatically skips tests if a
requirement fails.

Note that in places where we have a less strict test which should
always succeed (e.g. ioctl works or isn't available) the igt_assert
should be place before the igt_require with the more strict
requirements. Otherwise we'll skip a test instead of properly failing
it.

Convert a few users of igt_skip over to igt_require to showcase its
use.

v2: s/gem_check_/gem_require_/ so that we consistently use "require"
to indicate magic check that can call igt_skip. Imo hiding the
igt_require for feature checks is ok, but for more traditional assert
like use cases an explicit igt_require might be better.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>