Add tests for SkNextPow2

In a follow-up CL, I plan to replaces uses of GrNextPow2 with
SkNextPow2 (since there appears to be no Ganesh-specific logic
in the implementation). There aren't any existing unit tests for
the function, so this adds some.

Change-Id: Ie4e3231b2021664d2d321c4aa11e3fcecaf9e749
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/906956
Reviewed-by: Florin Malita <fmalita@google.com>