Test files for lone <sm/> and <em/>

Hi all,

Here are two test files for checking the readers see as invalid entries with a lone <sm/>, or <em/>.

David: Could you please add them to the test suite (in the "invalid" folder)? Thanks.

Note that a lone <em/> is likely to generate already an error in all readers as it would have no corresponding <sm> ID for its
startRef attribute.

Cheers,
-yves