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

Author: AlexanderChernikov

sys/modules/ktest/Makefile

@@ -0,0 +1,7 @@
+SYSDIR?=${SRCTOP}/sys
+.include "${SYSDIR}/conf/kern.opts.mk"
+
+SUBDIR=	ktest \
+	ktest_example
+
+.include <bsd.subdir.mk>

sys/modules/ktest/ktest/Makefile

@@ -0,0 +1,14 @@
+# $FreeBSD$
+
+PACKAGE=	tests
+
+SYSDIR?=${SRCTOP}/sys
+.include "${SYSDIR}/conf/kern.opts.mk"
+
+.PATH: ${SYSDIR}/tests
+
+KMOD=	ktest
+SRCS=	ktest.c
+SRCS+=	opt_netlink.h
+
+.include <bsd.kmod.mk>

sys/modules/ktest/ktest_example/Makefile

@@ -0,0 +1,13 @@
+# $FreeBSD$
+
+PACKAGE=	tests
+
+SYSDIR?=${SRCTOP}/sys
+.include "${SYSDIR}/conf/kern.opts.mk"
+
+.PATH: ${SYSDIR}/tests
+
+KMOD=	ktest_example
+SRCS=	ktest_example.c
+
+.include <bsd.kmod.mk>