Update default long-range option and expand test

This commit turns the long-range vdW correction _on_ for prmtops with NBFIX modifications.  It is already on for prmtops without NBFIX modifications, so this is the appropriate default to have.  I would have set that originally had I thought it was supposed to be supported.

I also added a test to test both the long-range and without the long-range correction.  The current long-range corrected test is commented out, since it would segfault, but it needs only be uncommented when that segfault is fixed.