Previously, this test could modify global state by changing
connection.settings_dict. This dict is a reference to the same dict as
django.db.connections.databases['default'], which is thus also changed.
The cleanup of this test would replace connection.settings_dic` with a
saved copy, which would leave the dict itself modified.
Additionally, create_test_db() would also modify these same dicts, as
well as settings.databases['default']['NAME'] by adding a "test_"
prefix, which is what can cause problems later.
This patch:
- makes a complete copy of the connection and work on that, to improve
isolation.
- calls destroy_test_db() to let that code clean up anything done by
create_test_db().
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