Skip switch fallthrough tests on Dawn

Skip SKSLSwitchWithLoops_GPU and SkSLSwitchWithFallthrough_GPU when
targeting Dawn as these shaders do not correctly translate to WGSL at
the moment.

Bug: skia:13922
Change-Id: If7314862fc8327d0bb81fe055d2171204f1bd16c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/605756
Reviewed-by: John Stiles <johnstiles@google.com>
Auto-Submit: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed