Rename shaders named `input` in test code.

`input` is a reserved word in GLSL. http://screen/85m4iRwvJRadKbV

Change-Id: I94ac0901964daa6c5fa01fc40ea3dbd733f821b7
Bug: skia:11115
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/454739
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed