test: ignore error looking for leaks

test: ignore error looking for leaks

The leaks counter frequently reports a leak between run 1 and 2 of the
tests. It turns out to be a Decimal Context whose items() method reports
no content. So the problem is related to Python, not psycopg, and it is
not an unbound leak.

Can't reproduce it in CI, but it happens on my laptop, since switching
to Ubuntu 22.04 with Python 3.10.