Clean up more tests wrt GrContext

Change-Id: I70189e316be19f28d70c2fae832a868101fd3329
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303998
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed