<NME> test-requirements.txt
<BEF> argon2-cffi>=16.1.0
factory-boy<3.3.0
httmock==1.2.5
testfixtures==4.7.0
tox

<MSG> fix test database setup

Django automatically renames test databases to 'test_%s' % settings.DATABASES[*]["NAME"], so modoboa_test became test_modoboa_test. There's no point creating the database in `before script` as the test suite creates it.

https://docs.djangoproject.com/en/1.11/topics/testing/advanced/#django.db.connection.creation.create_test_db

- target oldest currently supported database servers, travis currently defaults to PostgreSQL 9.2 which isn't supported anymore.

- move database dependencies out of travis.yml into test_requirements.txt and pin to minimum versions recommended by django.
  See https://docs.djangoproject.com/en/1.11/ref/databases/

- use MySQL root user for tests, we already use the root user (postgres) for PostgreSQL.

- fix typo in modoboa/test_settings for DB variable, default should be POSTGRESQL not POSTGRES

- enable MySQL strict mode (on by default on MySQL >= 5.7)
  See https://docs.djangoproject.com/en/1.11/ref/databases/#mysql-sql-mode

<DFF> @@ -3,3 +3,5 @@ mock==2.0.0
 httmock==1.2.5
 testfixtures==4.7.0
 tox
+psycopg2>=2.5.4
+mysqlclient>=1.3.3

fix test database setup