webui tests: fix assertion in test_subid.py
    
    The test wants to check the error related to an
    exception obtained inside a "with pytest.raises" instruction.
    The object is an ExceptionInfo and offers a match method
    to check the content of the string representation.
    Use this match() method instead of str(excinfo) which now
    returns
    '<ExceptionInfo NoSuchElementException() tblen=10>'
    
    Fixes: https://pagure.io/freeipa/issue/9282
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Mohammad Rizwan Yusuf <myusuf@redhat.com>