Using testing::RegisterTests lets us filter out the disabled configs.
This helps sharding the tests because some shards were getting a very
uneven distribution of real tests vs skipped tests. A couple shards
were running 20 minutes, while most were about 6 minutes. More work
might be needed to even that out.

Bug: angleproject:6090
Change-Id: Ibdf35677cfea86dfc374a2b18bb57cd9f289ddee
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2994726Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>