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.
Move code only needed for testing to test helper

Move code to clear root folder from Folder model to a test helper
method, as this code is only needed for testing.
Use helper method in folder tests

Use helper method to clear root folder in folder tests
Merge pull request #44 from Yardboy/mysql-test-updates

MySQL test updates