tests: Enable SAWarning warnings

We shouldn't be raising warnings from SQLAlchemy. Where we are
intentionally doing so, we should capture these warnings at the test
level. This requires some minor fixes.

Change-Id: I9d4512dc337153edc48a2cc3bf95ab2b31c39ccf
Signed-off-by: Stephen Finucane <stephenfin@redhat.com>