add_dune_test: Handle complex expressions in `CMAKE_GUARD`

The check `if(NOT ${condition})` does not behave as expected when
`condition` is a complex string expression such as `A OR B`. It does
however work when `condition` is a list.

Therefore convert `condition` to a list by calling `separate_arguments`.

Note that this change also adds missing parenthesis around `condition`
which also results in wrong results for complex expressions.

Closes issue #39.

cmake/modules/DuneTestMacros.cmake

@@ -228,7 +228,8 @@ function(dune_add_test)
   set(DOSOMETHING TRUE)
   set(FAILED_CONDITION_PRINTING "")
   foreach(condition ${ADDTEST_CMAKE_GUARD})
-    if(NOT ${condition})
+    separate_arguments(condition)
+    if(NOT (${condition}))
       set(DOSOMETHING FALSE)
       set(FAILED_CONDITION_PRINTING "${FAILED_CONDITION_PRINTING}std::cout << \"  ${condition}\" << std::endl;\n")
     endif()