Layer comps test

This pull request adds a basic test of the new layer comps feature in the Image Assets plugin. It also allows errors.txt files to be present in the expected and checked output of the assets plugin. It verifies the absence or presence of the file, as with any other file, but does not attempt to pixel-compare the files if they exist. 'Cause they ain't got no pixels, man.

At some point it would be nice to verify the *contents* of the error files, but that would require parsing the files somehow to avoid timestamp mismatches, which would require changing the format of these files, which I don't care to do now.

CC @joelrbrandt