only test slugs when padding is correct
only test slugs when padding is correct

The slug GM should only run if the experimental padding flag:
SK_EXPERIMENTAL_ADD_ATLAS_PADDING is defined.

Otherwise, there is just random pixel set based on atlas layout.

Change-Id: Ib5c2352b630d1ab4a24b5830ffcaa7f93666fd93
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/477757
Auto-Submit: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

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