[Cmake][Bugfix] add mpi flags to pamgtest without parmetis present

Mpi flags were not added to pamgtest if parmetis was not present.
This is of course unwanted behaviour. The patch adds pamgtest to
the mpi tests (in addition to being part of the parmetis tests)

dune/istl/paamg/test/CMakeLists.txt

 if(MPI_FOUND)
-  set(MPITESTS  galerkintest hierarchytest transfertest)
+  set(MPITESTS  galerkintest hierarchytest transfertest pamgtest)
 endif(MPI_FOUND)
 
 if(UMFPACK_FOUND)