Add a 5 minute timeout to python tests.

The CI times out after 10 minutes of no output. It's annoying if a
test hangs and is killed by the CI timeout, because you don't get
information about which test was running. Try to avoid that, by adding
a slightly smaller timeout in pytest itself. You can override it on a
per-test basis if needed, but let's try to keep our tests shorter than
that.

For the Postgres regression tests, use a longer 30 minute timeout.
They're not really a single test, but many tests wrapped in a single
pytest test. It's OK for them to run longer in aggregate, each
Postgres test is still fairly short.

pytest.ini

@@ -9,3 +9,4 @@ minversion = 6.0
 log_format = %(asctime)s.%(msecs)-3d %(levelname)s [%(filename)s:%(lineno)d] %(message)s
 log_date_format = %Y-%m-%d %H:%M:%S
 log_cli = true
+timeout = 300