Prevent unwanted logging while running tests

Previously, logging messages would output to std.out. when running
individual test files (instead of running the entire suite). To
prevent this, I've added a `for`-loop to `tests.helpers` that sets
the level of all registered loggers to `CRITICAL`.

The reason for adding this to `tests.helpers` is simple: It's the
most common file to be imported in individual tests, increasing the
chance of the code being run for individual test files.

A small downside of this way of handling logging is that when we are
trying to assert logging messages are being emitted, we need to set
the logger explicitly in the `self.assertLogs` context manager. This
is a small downside, though, and probably good practice anyway.

There was one test in `tests.bot.test_api` that did not do this, so
I have changed this to make the test compatible with the new set-up.

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