Enable plugins for jinja2 tests

##### Issue Type:

Feature Pull Request

##### Summary:

- Move boolean filters to `test_plugins/core.py` and enable users to write their own test plugins.
- Register all test plugins additionally as filter plugins to retain backwards compatibility.

Supersedes #8235
Discussed but never resolved [here](https://groups.google.com/forum/#!topic/ansible-devel/Oa6XSffcHJg)