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