Currently Swiftshader tests are skipped only if a swiftshader device is
requested. We should also skip swiftshader tests if the default GPU on
the system is swiftshader, as is the case in certain emulated systems.

Bug: angleproject:4626
Change-Id: I3ee83c43d35eb4f94b516e80689b241d53bbfb62
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2192090
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>