Add Bandit to tests in /integration

[mtrudel]

Intended to follow #5706. Depends on Phoenix moving its minimum Elixir version to 1.12, since Bandit has hard test-dependencies thereon.

[mtrudel]

These tests run quite a bit slower than previously because we're starting / stopping the underlying server via a setup block instead of a setup_all (ie: we're doing it for each test). Cowboy seems to have a stock 5 second waiting period at the end of every test as a result (previously, it only waited once, at the end of the entire file).

We don't really need to be doing this this way, but fixing it properly will be a bit more surgery than what's here. I'll get it fixed up.