[example] Fixup liddrivencavity test cmake skip

examples/liddrivencavity/CMakeLists.txt

 add_subdirectory(reference_data)
 dune_symlink_to_source_files(FILES "params_re1.input" "params_re1000.input" "run_and_plot.py")
-set(DUMUX_EXAMPLE_LDC_CMAKE_GUARDS "( HAVE_UMFPACK )")
+
+# set the test guard for the following tests (only enable test if UMFPack is available)
+set(DUMUX_EXAMPLE_LDC_CMAKE_GUARD "( HAVE_UMFPACK )")
 
 dumux_add_test(NAME example_ff_liddrivencavity
                SOURCES main.cc
                LABELS freeflow navierstokes example
-               CMAKE_GUARD DUMUX_EXAMPLE_LDC_CMAKE_GUARDS
+               CMAKE_GUARD ${DUMUX_EXAMPLE_LDC_CMAKE_GUARD}
                COMMAND ${CMAKE_SOURCE_DIR}/bin/testing/runtest.py
                CMD_ARGS      --script fuzzy
                              --files ${CMAKE_SOURCE_DIR}/test/references/test_ff_navierstokes_closedsystem_ldc_re1-reference.vtu
@@ -23,8 +25,11 @@ dumux_add_test(NAME example_ff_liddrivencavity_re1000
                              --command "${CMAKE_CURRENT_BINARY_DIR}/example_ff_liddrivencavity params_re1000.input
                              -Grid.Cells \"64 64\" -TimeLoop.TEnd 50")
 
-# test plot script (only if UMFPack is available and the tests above were actually run)
-if(DUMUX_EXAMPLE_LDC_CMAKE_GUARDS)
+# test plot script (only if the guard enables to TRUE, i.e. all preconditions for the above tests are met)
+dumux_evaluate_cmake_guard(DUMUX_ENABLE_LDC_PLOT_TEST
+                           CMAKE_GUARD ${DUMUX_EXAMPLE_LDC_CMAKE_GUARD})
+
+if (DUMUX_ENABLE_LDC_PLOT_TEST)
     dumux_add_test(NAME example_ff_liddrivencavity_plot
                    TARGET example_ff_liddrivencavity
                    COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/run_and_plot.py