[graphite] Add GraphiteTestContext

Bug: skia:12466
Change-Id: I90ebec9ee744de3822bd82d474858015d2490d72
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453636
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
7 files changed