Image subsets as part of the name instead of a separate source type.

This will hold us closer to the principle that the test name (and only the test
name) should correspond to expected output.  By the same reasoning, mix in the
number of subsets: if that changes, the expected output also changes.

BUG=skia:3255

Review URL: https://codereview.chromium.org/863723002
2 files changed