Use a Django database router, a TestCase mixin and thread local storage to allow unit tests to switch databases.

settings.py

# Detect if executed under test
TESTING = any(test in sys.argv for test in (
    'test', 'csslint', 'jenkins', 'jslint',
    'jtest', 'lettuce', 'pep8', 'pyflakes',
    'pylint', 'sloccount',
))

if TESTING:
    # If testing, move the default DB to 'mysql' and replace it
    # with a SQLite DB.
    DATABASES['mysql'] = DATABASES['default']
    DATABASES['default'] = {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': ':memory:',
    }
    # Install our router so that unit tests can choose a DB.
    DATABASE_ROUTERS = ('myapp.tests.TestUsingDbRouter', )

tests.py

import threading

from django.db import DEFAULT_DB_ALIAS
from django.test.testcases import TestCase


_LOCALS = threading.local()


def set_test_db(db_name):
    "Sets the database name to route to."

    setattr(_LOCALS, 'test_db_name', db_name)


def get_test_db():
    "Get the current database name or the default."

    return getattr(_LOCALS, 'test_db_name', DEFAULT_DB_ALIAS)


def del_test_db():
    "Clear the database name (restore default)"

    try:
        delattr(_LOCALS, 'test_db_name')
    except AttributeError:
        pass


class TestUsingDbRouter(object):
    "Simple router to allow DB selection by name."

    def db_for_read(self, model, **kwargs):
        return get_test_db()

    def db_for_write(self, model, **kwargs):
        return get_test_db()


class UsingDbMixin(object):
    "A mixin to allow a TestCase to select the DB to use."

    multi_db = True
    using_db = None

    def setUp(self, *args, **kwargs):
        super(UsingDbMixin, self).setUp(*args, **kwargs)
        set_test_db(self.using_db)

    def tearDown(self, *args, **kwargs):
        del_test_db()
        super(UsingDbMixin, self).tearDown(*args, **kwargs)


class MySQLTestCase(UsingDbMixin, TestCase):
    "A unit test to run against the 'mysql' database."

    using_db = 'mysql'

    def test_mysql_something(self):
        pass  # TODO: test something specific to MySQL