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>