External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
This assertion was silently a no-op for years, and it apparently fails.
So since we were not even testing it before, it is at least no worse to
just remove it.
AFAICT, `mock.has_calls` *never* existed. It only started emitting a
warning recently. And for some reason I only see this crop up on
Windows? Truly mysterious.
