Update SkSLTest to check a 2x2 surface.
Update SkSLTest to check a 2x2 surface.

Derivative tests don't really make intuitive sense in a 1x1 surface.
This should give the derivatives a better workout.

Change-Id: I09e5ff2b7815d42d1874e5deef360e9bc2e83aff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/449195
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>
Reviewed-by: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>

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