[cmake][Release] Allow downstream modules to disable the testing magic

Some downstream modules like pdelab and dumux already abandoned the
current bahaviour of building tests on make test. The testing infrastructure
does throw them error messages during make test though. This commi introduces
the switch DUNE_DISABLE_TEST_MAGIC which may be set by downstream modules
to disable the magic. To see how to switch a downstream module, see:

http://conan2.iwr.uni-heidelberg.de/git/pdelab/dune-pdelab/commit/dc5ef4b39f5be5690457f0bbfe008b1f45d5fd98

cmake/modules/DuneMacros.cmake

@@ -882,7 +882,9 @@ endif()
     configure_file(config.h.cmake ${CMAKE_CURRENT_BINARY_DIR}/config.h)
   endif("${ARGC}" EQUAL "1")
 
-  test_dep()
+  if(NOT DUNE_DISABLE_TEST_MAGIC)
+    test_dep()
+  endif()
 
   include(CPack)
 

cmake/modules/DuneTestMacros.cmake

@@ -47,8 +47,10 @@ endmacro(get_directory_test_target _target _dir)
 # E.g. for dune/istl/test the target will be dune_istl_test.
 #
 macro(add_directory_test_target _target)
-  get_directory_test_target(${_target} "${CMAKE_CURRENT_BINARY_DIR}")
-  add_custom_target(${${_target}})
-  dune_common_script_dir(SCRIPT_DIR)
-  configure_file(${SCRIPT_DIR}/BuildTests.cmake.in BuildTests.cmake @ONLY)
+  if(NOT DUNE_DISABLE_TEST_MAGIC)
+    get_directory_test_target(${_target} "${CMAKE_CURRENT_BINARY_DIR}")
+    add_custom_target(${${_target}})
+    dune_common_script_dir(SCRIPT_DIR)
+    configure_file(${SCRIPT_DIR}/BuildTests.cmake.in BuildTests.cmake @ONLY)
+  endif()
 endmacro(add_directory_test_target)