Add makeNotCurrent to TestContext

I find this useful for managing which context is active on threads.

Change-Id: I655955a2351f273751681f080190ca684d29db4e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270642
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
14 files changed