Try to stop unit tests from acting up by using smaller files

Jenkins keeps saying "libgomp: Thread creation failed: Resource
temporarily unavailable" which google says is mem limit related,
however locally the test works even with the shell mem limit
set to a mere 50 MB (but I'm using newer image magick).

Anyways, this can't hurt.

Change-Id: I85de5ebe331e56d79a2e9e91a38ba1231eddab8a