Consolidate sample images #6
Description
Tests written as part of #3 generate the same image in different formats, in multiple tests (via shell redirection, decoded from base64 output, written directly to a file via --output, etc.). Change shouldMatch to take an optional second argument, so we can merge images that should be identical into a single file (testing.{jpg,png,bmp,jpg-64,png-64,bmp-64}).