Tighten _check_compose regexes to catch more invalid composes
    
    As the FIXME notes mentioned, this has been broken for a long
    time but I didn't notice as the test itself was broken and never
    reached the checks that should have failed. We need to bound the
    regexes to catch wrong characters at the end (and start) of the
    compose identifiers.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>