Also moves ANGLE_SKIP_TEST_IF to a header shared between end2end and
perf tests.

BUG=chromium:997674

Change-Id: I7c5968e6d861c4bff703ddc6ae0e4e021e47061f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1768374
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>