Miscellaneous test skips and fixes for iOS.

Bug: angleproject:5417
Bug: angleproject:5491
Change-Id: Id0785e6243949fc756e4d7923dbbe77a411052f3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2606656Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>