Fix incorrect error when testing is disabled.
Fix incorrect error when testing is disabled.

When building with MPI and testing is disabled (which happens when
building on supercomputers), VTK fails to build because of missing
MPIEXEC. That's wrong. MPIEXEC is only needed if testing is enabled.
Fixed the error message accordingly.

sumedhasingla/VTK,mspark93/VTK,sankhesh/VTK,mspark93/VTK,SimVascular/VTK,keithroe/vtkoptix,gram526/VTK,mspark93/VTK,gram526/VTK,jmerkow/VTK,jmerkow/VTK,jmerkow/VTK,jmerkow/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sankhesh/VTK,jmerkow/VTK,mspark93/VTK,sankhesh/VTK,gram526/VTK,sankhesh/VTK,mspark93/VTK,gram526/VTK,mspark93/VTK,SimVascular/VTK,SimVascular/VTK,sankhesh/VTK,mspark93/VTK,sumedhasingla/VTK,gram526/VTK,sumedhasingla/VTK,SimVascular/VTK,sankhesh/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,gram526/VTK,keithroe/vtkoptix,jmerkow/VTK,sankhesh/VTK,mspark93/VTK,jmerkow/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sankhesh/VTK,gram526/VTK,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,keithroe/vtkoptix,jmerkow/VTK,gram526/VTK,SimVascular/VTK,SimVascular/VTK