Update aprun launchcmd helper script to support both moab and PBSPro.

Refactors `util/test/launchcmd-for-aprun-launcher` to support both moab and PBSPro flavors of pbs. To do this, I refactored most of the functionality into a single abstract class, `AbstractJob` and then implemented `PbsProJob` and `MoabJob` classes that inherited from `AbstractJob`. The three things that needed to differ (and are implemented on the sub classes) are: 1) the naming of the hostlist resource, 2) the job name must be 15 chars or less in PBSPro, and 2) the output and arguments to qstat are different.

I verified this works on jupiter (XC w/ moab) and kay (XC w/ PBSPro) by running the following:

```bash
cd test/release/examples/primers
start_test --launchcmd '$CHPL_HOME/util/test/launchcmd-for-aprun-launcher' .
```

This will also support XE testing with PBS.