Refactor SdssCentroid test to better use test utilities

The first several lines of the previous version could be accomplished
by simply inheriting from meas.base.tests.AlgorithmTestCase, which
additionally puts that code in setUp(), allowing it to be reused
for other test functions.

Also fixed non-conformant variable names.
Catch exception in SdssCentroid to replace log warning with set flag
Add edge flag for NaiveFlux, to account for cases when aperture doesn't fit in frame