Run migrations on test DB with alembic --tag=test.

Summary:
You can now run alembic migrations on the test database dump with the following
sequence of commands:

mysql test < tests/data/base_dump.sql
alembic upgrade head --tag=test
mysqldump test > tests/data/base_dump.sql

Hopefully this will make it easier to keep the test dump current, and also help
us sanity-check our migration scripts.

Test Plan: I did this with a pending migration script I have and it worked.

Reviewers: spang

Reviewed By: spang

CC: kav-ya, mg

Differential Revision: https://review.inboxapp.com/D69