Update Metal unit test blacklist after adding new promise image tests

Bug: skia:8736
TBR: robertphillips@google.com

Change-Id: I13b49909d62b0f4c685ded3345e9e9cd35363554
Reviewed-on: https://skia-review.googlesource.com/c/188304
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
2 files changed