Fix the vtk.diff_serial_mesh_parallel test

Change the partitioner for this test, and enable it on GCC.

For some reason, the default (Metis) partitioner partitioned this 2x2
mesh differently on Linux vs. Mac, i.e.
```
-------------
|  0  |  1  |
-------------
|  0  |  1  |
-------------
```
vs.
```
-------------
|  1  |  0  |
-------------
|  1  |  0  |
-------------
```
Refs #3689.