pkgbase: Put atf_python files in FreeBSD-tests

Sponsored by: Beckhoff Automation GmbH & Co. KG

Testing: add framework for the kernel unit tests.

This changes intends to reduce the bar to the kernel unit-testing by
introducing a new kernel-testing framework ("ktest") based on Netlink,
loadable test modules and python test suite integration.

This framework provides the following features:
* Integration to the FreeBSD test suite
* Automatic test discovery
* Automatic test module loading
* Minimal boiler-plate code in both kernel and userland
* Passing any metadata to the test
* Convenient environment pre-setup using python testing framework
* Streaming messages from the kernel to the userland
* Running tests in the dedicated taskqueues
* Skipping or parametrizing tests

Differential Revision: https://reviews.freebsd.org/D39385
MFC after: 2 weeks

netlink: connect netlink tests to the build

Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D37708

testing: Add basic atf support to pytest.

Implementation consists of the pytest plugin implementing ATF format and
a simple C++ wrapper, which reorders the provided arguments from ATF format
to the format understandable by pytest. Each test has this wrapper specified
after the shebang. When kyua executes the test, wrapper calls pytest, which
loads atf plugin, does the work and returns the result. Additionally, a
separate python "package", `/usr/tests/atf_python` has been added to collect
code that may be useful across different tests.

Current limitations:
* Opaque metadata passing via X-Name properties. Require some fixtures to write
* `-s srcdir` parameter passed by the runner is ignored.
* No `atf-c-api(3)` or similar - relying on pytest framework & existing python libraries
* No support for `atf_tc_<get|has>_config_var()` & `atf_tc_set_md_var()`.
Can be probably implemented with env variables & autoload fixtures

Differential Revision: https://reviews.freebsd.org/D31084
Reviewed by: kp, ngie