Draft: Run runners in parallel

I tried running tests in parallel using pg_virtualenv to isolate databases.

There are two issues I found so far:

1. If many pg_virtualenv are started at the same time, they race and have a tendency to pick the same port, so some tests will end up running on the same database after all
2. If a test stops with Django asking Type 'yes' if you would like to try deleting the test database 'debusine-test-signing', or 'no' to cancel:, running in parallel means we are not interactive and we don't have a chance to say yes. Possibly --no-input may help with that.

This is getting way out of scope, so I'm stopping here. Anyone can feel free to pick this up if they get inspired