test(db): fixing db.removeUser tests for mysql

- scope must be an array
- clientId and userId must be buffers
- userId is 16 bytes

Also, I moved the db setup for the test into a before() block, to make it
easier to see that the test is actually about db.removeUser()
Merge pull request #172 from mozilla/remove-user-failure

test(db): fixing db.removeUser tests for mysql
r=rfk