[tests] return 0 instead of 77 if alugrid is not present
ctest reports skip tests ( return code 77 ) as failed. This results in a failing nightlybuild if alugrid is not present. Forther discussions on the return code 77 with cmake can be found here:
http://public.kitware.com/Bug/view.php?id=8466 flyspray/FS#1538 (closed)