Features not supported seem:

- isolation level (always serializable)
- client encodings
- notices (maybe there is a way to generate them)
- 2 phase commit
- reset (because of the lack of transaction deferrable)
- backend pid

If the CREATE TABLE statement fails, the setup would fail
without committing or rolling back the active transaction, so the
transaction would hold onto its resources indefinitely.

Normally, the transaction would be closed when the connection is closed
in the `tearDown` function. However, `tearDown` is not called if there
was an error during `setUp` ([as specified by the `unittest` docs](https://docs.python.org/3/library/unittest.html#unittest.TestCase.tearDown)), so
we need to handle this case specially.

Previously, this test had a bug, because if the CREATE TABLE statement
failed, the setup would fail without committing or rolling back the
active transaction.

I don't know why it returns 0 instead of the right value. At least it
doesn't segfault, so don't skip the test altogether.

The test is unrelated to this branch: will cherry-pick elsewhere (if I
remember it...)

It won't work on windows if it's in the script: failing with errors
such as:

    AttributeError: 'module' object has no attribute 'process'

or:

    Can't get attribute 'process' on <module '__main__' (built-in)>

Unrelated processes close the FD of the connection. This happens in
Python 3.6 but not 2.7. Let's see if travis shows where else it fails...