gfs2-utils tests: Skip unit tests if check is not found
    
    The check package (check-devel in Fedora) is required for building the
    unit tests. When the configure script doesn't find check, it prevents
    the unit tests from being built, but the test suite still tries to run
    the unit tests. Introduce a wrapper m4 macro GFS_UNIT_TEST to set up
    unit test test groups which will be skipped if check was not found in
    the configure stage.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>