[CMake] Use guards to conditionally set MPITESTS, otherwise cmake

aborts.

This differs from the way we handle tests requiring MPI compared to
dune-common. Maybe we should discuss which way we want to use in Dune in
general: HAVE_* macros or build guards in Makefile.am/CMakeLists.txt.

[[Imported from SVN: r1603]]

dune/istl/test/CMakeLists.txt

-set(MPITESTS vectorcommtest matrixmarkettest matrixredisttest )
 set(NORMALTEST basearraytest matrixutilstest matrixtest
   bvectortest vbvectortest bcrsbuildtest matrixiteratortest mv
   iotest scaledidmatrixtest seqmatrixmarkettest)
@@ -12,6 +11,10 @@ if(HAVE_SUPERLU)
     overlappingschwarztest)
 endif(HAVE_SUPERLU)
 
+if(HAVE_MPI)
+  set(MPITESTS vectorcommtest matrixmarkettest matrixredisttest)
+endif(HAVE_MPI)
+
 set(ALLTESTS ${MPITESTS} ${NORMALTEST} ${PARDISOTEST} ${SUPERLUTESTS})
 message("ALLTESTS=${ALLTESTS}")
 message("MPITESTS=${MPITESTS}")