These tests make sure that a depth- or stencil-only format that is
emulated with a depth/stencil one does not affect clear/draw in such a
way that it would expose the hidden aspect.

By nature of the emulation, it's not trivial to read back the hidden
aspect.  This test simply makes sure the hidden aspect doesn't affect
rendering, but is not a definitive test of correctness.

Bug: angleproject:2815
Change-Id: I158937faa019487f6fa08154406c152753a1934b
Reviewed-on: https://chromium-review.googlesource.com/c/1314188
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>