[test] Shorten prefix string in test_rql_role_with_unknown_vid
With Python3.7, this test fails as the error message returned by
rest_publish() is:
<p>an error occurred while interpreting this rql directive: ObjectNotFound('toto')</p>
while we previously assumed a comma after 'toto'.
Fixing this by only shortening the prefix used in out.startswith().