Update launchcmd script to support SLURM.

It turns out using the prediff method for slurm testing was not sufficient for tests that generate output to
files (and then use .catfiles) (e.g. lulesh).

The details are: .catfiles are evaluated before the prediff is called and after the "execution" completes.
For slurm testing using the sbatch mode, "execution" completes almost immediately and then the prediff
waits until the job execution has actually completed to continue on. Since catfiles was running before
the job actually finished, this caused a problem.

So, the launchcmd script is updated to support slurm (in addition to PBSPro and Moab). This will allow
the wait to happen during normal exection. Then the normal catfiles, prediffs, and other test mechanisms
will work as they normally do.

Since the prediff script is not necessary anymore, it is also removed (all of its functionality is in
launchcmd, though, so it is not lost!).

### TODO
* [ ] verify slurm change work (#361)
* [ ] verify these changes still work for moab and pbspro flavors of qsub
* [ ] verify these changes work for slurm!