Move 'shader' usage tests to an SkSL golden file

Also adds tests of non-uniform shader declarations. These are currently
allowed, but will be detected as an error in the next CL.

Bug: skia:11374
Change-Id: I3fee0a0c97ae590f7bc6952cb367f7e94436b891
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393080
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed