Remove leftover naive datetimes in tests

Oddly enough, the warnings were only thrown under Python 2.6. Should
investigate!
Test suite: Treat RuntimeWarnings as errors

This ensures we catch e.g. the "naive datetime" RuntimeWarning that
Django throws.
Ideally, we'd fail on all Warnings, but a quick look at implementing
this showed that there's quite a few warnings we have no control over.
Might not be worth the effort of constantly whitelisting them.