Cleaning python testing macros.

When attemping to add tests using vtk_add_test_python_mpi(), issues were
spotted in the vtkTestingMacros code. This commit addresses those
issues.

VTK_PYTHON_EXE is no longer globally defined in CMakeLists.txt.
vtk_add_test_python_mpi() and vtk_add_test_python() assume a good
default is none is set (this allows for applications like ParaView to
use these macros while overriding the executable to run).

We use generator expressions to point to the default
vtkpython/pvtkpython executables which works better when MPI
(mpiexec/mpirun) is being used to launch the test executable.