Update comparison in BackendAllocation tests

Change-Id: I11760741251b03dfb457d4b07ef3cb6a560c6f7b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240683
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed