Clean up some GrContext in tests

Change-Id: I58372d0691fe6a348288a6076d78d3455288192a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303660
Reviewed-by: Adlai Holler <adlai@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
12 files changed