tests: Clean up lost test guilds

[silasary]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

I noticed that #675 was failing because we hit the ten guild limit.
This should only have happened if we were either:

• Running more than 10 simultaneous test jobs at the same time.
• Or tests have at some point in the past failed so catestrophically that they didn't delete their guild at the end of the run.

This PR adds a (rather naive) check for an existing test guild that's more than a day old, and if so, deletes it.
Definitely could be optimized or improved, but eh, this is good enough for our purposes.

Changes

• Adds the ability to to load tokens from a .env file (mostly for my own convenience)
• Adds a naive cleanup procedure to the top of the guild fixture.

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code