Clean up some uses of GrContext in tests

Change-Id: Ic1f2f4edb8bfa27fa28f5da22575e1280a51d37b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303641
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Adlai Holler <adlai@google.com>
4 files changed