tests: ensure we don't see database errors during duplicate insert

tests: ensure we don't see database errors during duplicate insert

Currently, the parser causes IntegrityErrors while inserting duplicate
patches; these tend to pollute database logs.

This change adds a check, which currently fails, to ensure we do not
cause errors during a duplicate patch parse.

Conflicts:
	patchwork/tests/test_parser.py

NOTE(stephenfin): Conflicts are once again due to import reordering. In
addition, we have to skip these tests on Django 1.11 since the
'connection.execute_wrapper' context manager was first added in Django
2.0 [1].

[1] https://docs.djangoproject.com/en/dev/releases/2.0/#models

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Stephen Finucane <stephen@that.guru>
[stephenfin: Add 'expectedFailure' marker to keep all tests green]
(cherry picked from commit a60e75e2c6897fd262ec95a35e0e94b9027c11d4)