Add test_sqlite_test_name_used

Ref: https://github.com/pelme/pytest_django/issues/103
Fix `--reuse-db` for SQLite (memory) properly

This was fixed "by accident" in c231f53f, because `:memory:` would not
exist as a file.

I am a bit unsure about if this breaks xdist functionality, but it seems
that all existing tests pass.

Fixes https://github.com/pelme/pytest_django/issues/103
Fix "ImproperlyConfigured" on master with TEST_NAME and TEST settings

The full error:

> ImproperlyConfigured: Connection 'default' has mismatched TEST and
TEST_* database settings.
Stab at fixing TestSqliteWithXdist: use conn.creation._get_test_db_name
Add "sqlite_file" to settings for tox/travis