Add helper uniforms to Runtime Effect tests.

This lets us use descriptive names like `colorRed` and `colorGreen`
instead of `half4(1,0,0,1)` and `half4(0,1,0,1)`. It also lets us use
actual unknown values instead of synthesizing sorta-kinda-unknowns by
calling sqrt.

Change-Id: I61481c33b7ff42182955777b05cfa5fcc13e0efc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/359567
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
24 files changed