Include more functional migrator tests

[wouterj]

E.g. to catch errors like #37

[theofidry]

I think this can be detected by having a functional test doing:

• 1 migration to create a table => check the table existence
• populate the table => check the number of persisted entities
• reset the migration => check the table (non) existence

[wouterj]

At the moment (1) and (2) is already covered: https://github.com/wouterj/WouterJEloquentBundle/blob/master/tests/Functional/MigrationsTest.php

So we just need to extend this with (3).