Warn about `__init__.py` in tests

Rule request

Description

It is the best practice to remove __init__.py from tests folder. https://docs.pytest.org/en/latest/goodpractices.html#tests-outside-application-code

Rationale

So, this linter should warn users about incorrect setup. It is probably a good idea to make this configurable:

1. tests path
2. style: with __init__.py or without

[sobolevn]

1. When you have duplicate test names

[webknjaz]

Having this configurable is a good idea. While pytest emulates the presence of __init__.py, there are cases when this may break (especially when you also have to support Python 2).

I personally like not having __init__.py, but FTR here’s some corner cases that I met:

1. various tooling does not support/understand implicit namespaces or such support is implemented poorly
2. when the custom import machinery is implemented, it may conflict with what pytest does and it has to be disabled (this is the case I saw in Ansible, in particular)

It’s probably worth mentioning the presence of the exceptional cases in the doc for the rule, once implemented.