fix build failure regarding #110

disable plugin tests (that fail because cframework is missing) when BUILD_TESTING is OFF
fix #32 (was still marked as MEMLEAK)
have MEMLEAK as option for add_plugintest (needed for supporting BUILD_TESTING BUILD_SHARED)
temporary fix for disabling c++ tests

better solution would be to port them to gtest (also for plugin testing)

improves #110:
the cmake statement as given there now outputs:
Running tests...
No tests were found!!!