MySQL test updates

Changes to accommodate differences between MySQL and SQLite3 in the test
suite. MySQL does not reset auto-increment number on rollback, which was
causing failing tests when Folder.root did not match the most recently
created root folder. Also reduced limit on ShareLink.emails from 256 to
255 characters to accommodate MySQL string limits.

As discussed in: https://github.com/mischa78/boxroom/issues/42