Add test for searching by subject only

@reedstrm: I know you recently created a pull request for searching by subject:

- make subject a limit search - allow linit-only searchs
- make subject a limit search - allow linit-only searchs
- Merge branch 'limit-only-search' of github.com:Connexions/cnx-archive into limit-only-search
- make the subject/tag limit join conditional, to avoid excess rows on non-limit searches
- more created/revised fixes in tests
- Merge pull request #130 from Connexions/limit-only-search

I tried to write a test that would break before your changes and pass after.  But the test I wrote already passes before your changes.  That probably means that I'm testing the wrong thing.  Could you have a look and let me know?