Fix so that valgrind checks actually works with CMake.

The valgrind tests where not run on the suites.

And valgrind was always returning 0 so set an explicit exit code on exit. I also had forgotten to change the name of TEST_WITH_VALGRIND to JANSSON_TEST_WITH_VALGRIND so that the tests would never use valgrind.
Merge pull request #160 from JoakimSoderberg/cmake_memcheck_fix

Fix so that valgrind checks actually works with CMake.