Improve tests speed

[liZe]

This pull request includes two commits.

1. Use a very short timeout if we want to exit when the Redis queue is empty.
2. Use simple code to check that no table is in database (only used for tests).

[liZe]

Tests were passing on my computer, but not on GitHub Actions. Why? The idea of using a short timeout was nice, but it didn’t work. Why? No error in the logs but no line added in the database, a stupid problem impossible to debug. Why?

Because we use Redis 5, and floats for timeouts are only accepted since … Redis 6.

So, after the third way, here is the fourth way: keep brpop for endless loops, and use rpop when we want to exit early. This solution is a bit more complicated, but the code is easy to understand, and it’s actually The Good Way™ to avoid a mandatory timeout.

Both cases are actually used in tests: we generally use early exits when we want to test the Redis queue, but we also test signals to stop Azafea running forever. We keep our 100% coverage 🎉.

Bonus: floats are now allowed for timeouts in Redis typing, if we ever want to use them: python/typeshed#4952.

Tests on my computer:

• master: ~400s
• first commit: ~240s (65% faster)
• second commit: ~220s (10% faster)

And tests are 2 times faster on GitHub actions.