Lift some test suppressions for bugs that have been fixed in the
NVIDIA driver a good while ago.

Also improve code style in some suppressions - prefer IsOSX() and
IsLinux() to using ifdefs.

BUG=angleproject:1305
TEST=angle_end2end_tests

Change-Id: Ia069f5b9696000c901956d2952801d5f5c339a11
Reviewed-on: https://chromium-review.googlesource.com/518168Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>