Remove usage of sqrt() as an optimization barrier in tests.

In the majority of cases, a uniform is an equally good substitute, and
replacing `sqrt(N)` with `unknownInput` actually makes the test clearer.

Change-Id: I7bcb477571972d7aa2ce8c49b3674471f7310748
Bug: skia:12034
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411306
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
72 files changed