Improve GrResourceAllocator test infrastructure

Adding new tests in an upcoming CL. This sets the stage for that.

I also outlined the Register ctor so that the diff in the next CL is
more readable.

Bug: skia:10877
Change-Id: I604f1ce811cddf1d1105716f114e27ce12bf8249
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/394998
Commit-Queue: Adlai Holler <adlai@google.com>
Auto-Submit: Adlai Holler <adlai@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
3 files changed