This also allows us to remove all the one off Fence code that we
implemented in all the backend TestContexts
Change-Id: I9ff7ba4690cf3f19a180f51fc510991a112bb62c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272456
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
