Raise warning when using SQLite3 in-memory test database with --reuse-db

Currently I've running several unittests with py.test that needs a database. The tests working fine.
But I#ve running with --reuse-dband the following database settings:

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': 'db.sqlite3'
    }
}

The Tests will work fine, but the databse db.sqlite3 won't getting created / the file is missing while the tests run / after the tests.
I use django unittests and run py.test with the following: py.test -v -s --ds=envisia.test.settings --reuse-db also I'm running on Python 3.4.1, pytest 2.6.1 and Django 1.7rc2.
On PostgreSQL the database will be created and will exists..

Edit: Also PostgreSQL tests run faster even when I run sqlite3 on :memory: which is somehow strange?! But I think that's cause of PostgreSQL --reuse-db works.

Reproduce the issue with the shown SQLite settings and py.test -v -s --ds=envisia.test.settings --reuse-db command, then trace the plugin's handling of SQLite and --reuse-db. Done means the incompatible in-memory or file-backed behavior is detected and a clear warning is raised; add or update a regression test for that case.