minor typo correction
Changed testing classes to take advantage of Tester.run returning a
boolean.
Pulled out the pixel-for-pixel comparison into a separate class.
This was originally done in order to use checkEquivalent() rather
than checkExpect, but when I changed my mind on that (so, for example,
two objects with fields of type WorldImage could be compared using pixel
equivalence), I kept the LooksTheSame class for organizational purposes.