Disable SkSL testing on Dawn + Ganesh.

This ends up testing the SPIR-V backend and Tint's SPIR-V reader.
We already test our SPIR-V backend against native hardware, and
the Tint SPIR-V reader has a number of known limitations/bugs.

Change-Id: I73d872e7b50b0b13dd3dc902fcc96b671c1c555d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/744257
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
7 files changed