Limit GrClipStackClip tests to only exist when relevant

As part of the new architecture I envision not compiling in stuff like
the GrClipStackClip when in NGA-mode. This ran/runs into trouble with
the unit tests.

Change-Id: I922dcf873c4bb1a3abfd2ce519f3dbf68e053770
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/392839
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed