CCode: Use `CPP_TEST` instead of `add_plugintest`
-if (ADDTESTING_PHASE)
- add_plugintest (ccode CPP LINK_ELEKTRA elektra-kdb elektra-plugin)
-endif (ADDTESTING_PHASE)
-
-add_plugin (ccode CPP SOURCES ccode.hpp ccode.cpp coder.hpp coder.cpp TEST_README TEST_REQUIRED_PLUGINS tcl base64)
+add_plugin (ccode CPP ADD_TEST CPP_TEST SOURCES ccode.hpp ccode.cpp coder.hpp coder.cpp TEST_README TEST_REQUIRED_PLUGINS tcl base64)
