phpunit: Set much smaller defaults for RandomImageGenerator

This class is not used very often:
https://codesearch.wmcloud.org/search/?q=RandomImageGenerator
But in these tests it's a serious bottleneck. For no good reason. We
don't really care how an image looks like during a test. Nobody will
ever see it. Not even when a test fails.

Bug: T225730
Change-Id: I7f3d586144ecdf480c773dc5e8b10f1ad4df8861