Fix test RequestNull bug

1 of the tests (test RequestNull) failed on Mac OS X 10.9 with Open MPI
1.6.5. From my reading of the standard, MPI_Test with a null
request (MPI_REQUEST_NULL) returns an "empty" or undefined status. There
is no guarantee about the error associated with the status except that
it should return 0 for calls such as get_count, get_element etc.

This fix uses the CheckStatus call which ignores the error associated
with the status.