[CMake] Add MPITESTS and PARMETISTESTS only conditionally to ALLTESTS.

[[Imported from SVN: r1616]]

dune/istl/paamg/test/CMakeLists.txt

-set(MPITESTS  galerkintest hierarchytest transfertest)
-set(PARMETISTESTS pamgtest pamg_comm_repart_test)
+if(MPI_FOUND)
+  set(MPITESTS  galerkintest hierarchytest transfertest)
+endif(MPI_FOUND)
+
+if(ParMETIS_FOUND)
+  set(PARMETISTESTS pamgtest pamg_comm_repart_test)
+endif(ParMETIS_FOUND)
+
 set(NORMALTESTS amgtest graphtest kamgtest)
 set(ALLTESTS ${MPITESTS} ${PARMETISTESTS} ${NORMALTESTS})