The test methods were duplicated between 3d and 2d array tests.  This
change creates common methods that can be called by the two test sets.
Additionally:

- Input colors are symbolized as they are repeated multiple times.
- The color used for unmultiply-alpha test is slightly tweaked to avoid
  generating exactly x.5 values with undefined rounding.

Bug: None
Change-Id: If0cf694017724a72976d3d5cb0f54f8c49a0502a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2246319
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>