Previously our output would spam multiple lines by checking equality
for each color component. Instead, use a helper struct with a stream
operator to clean up the output. This makes it much more legible.

New output style:

  Expected: (0, 255, 0, 255)
  Actual: angle::MakeGLColor(255, 0, 127, 255)
  Which is: (255, 0, 127, 255)

BUG=angleproject:1290

Change-Id: Ieea018ab6c149a20b5fd74d74c972118b63a4b5e
Reviewed-on: https://chromium-review.googlesource.com/323441Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>