Speed up romanisim tests

#261 introduced new ePSF capabilities and added corresponding tests.  I have the impression that the tests now take ~3x longer to run.  This is more than I would expect.

Some part of this may be because the new ePSF stamps are much larger and go quite far into the PSF wings.  If that is the primary cause, we should reduce the stamp size to match the STPSF stamp size.