Fix asserts in Metal tests.

Bug: skia:8243
Change-Id: I5d976e2740bd0348e1c71cea2cda05d93bc004e9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201397
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
9 files changed