Move "{IO/Parallel => Parallel/MPI}/Testing/Tcl/TestPProbe.tcl" because
the test depends on pvtk which exists only when the vtkParallelMPI
module is enabled.  Teach prtImageTest.tcl to use "package require vtk"
just as was done for rtImageTest.tcl by commit f095f7ee (Added in some
Tcl rendering tests, 2012-04-11).  Add Parallel/MPI Tcl test
infrastructure and include TestPProbe.  Remove TestPProbe from its old
test location in IO/Parallel.

Change-Id: I49c2a72762a5094e44c8ad964a309ca97cec8ca4