[CMake] Implement a COMPILE_ONLY option for dune_add_test

Set if the given test should only be compiled during `make build_tests`,
but not run during `make test`. This is useful if you compile the same
executable twice, but with different compile flags, where you want to assue that
it compiles with both sets of flags, but you already know they will produce the
same result.

cmake/modules/DuneTestMacros.cmake

@@ -105,6 +105,15 @@
 #       this in combination with the MPI_RANKS parameter, the call to mpi will still be
 #       wrapped around the given commands.
 #
+#    .. cmake_param:: COMPILE_ONLY
+#       :option:
+#
+#       Set if the given test should only be compiled during :code:`make build_tests`,
+#       but not run during :code:`make test`. This is useful if you compile the same
+#       executable twice, but with different compile flags, where you want to assue that
+#       it compiles with both sets of flags, but you already know they will produce the
+#       same result.
+#
 #    This function defines the Dune way of adding a test to the testing suite.
 #    You may either add the executable yourself through :ref:`add_executable`
 #    and pass it to the :code:`TARGET` option, or you may rely on :ref:`dune_add_test`
@@ -143,7 +152,7 @@ endif()
 
 function(dune_add_test)
   include(CMakeParseArguments)
-  set(OPTIONS EXPECT_COMPILE_FAIL EXPECT_FAIL SKIP_ON_77)
+  set(OPTIONS EXPECT_COMPILE_FAIL EXPECT_FAIL SKIP_ON_77 COMPILE_ONLY)
   set(SINGLEARGS NAME TARGET)
   set(MULTIARGS SOURCES COMPILE_DEFINITIONS COMPILE_FLAGS LINK_LIBRARIES CMD_ARGS MPI_RANKS COMMAND)
   cmake_parse_arguments(ADDTEST "${OPTIONS}" "${SINGLEARGS}" "${MULTIARGS}" ${ARGN})
@@ -224,7 +233,7 @@ function(dune_add_test)
 
   # Add one test for each specified processor number
   foreach(procnum ${ADDTEST_MPI_RANKS})
-    if(NOT "${procnum}" GREATER "${DUNE_MAX_TEST_CORES}")
+    if((NOT "${procnum}" GREATER "${DUNE_MAX_TEST_CORES}") AND (NOT ADDTEST_COMPILE_ONLY))
       if(NOT ${procnum} STREQUAL "1")
         set(ACTUAL_TESTCOMMAND ${MPIEXEC} ${MPIEXEC_PREFLAGS} ${MPIEXEC_NUMPROC_FLAG} ${procnum} ${ADDTEST_COMMAND} ${MPIEXEC_POSTFLAGS})
         set(ACTUAL_NAME "${ADDTEST_NAME}-mpi-${procnum}")