MFC r261081, r261133, and r261615

r261081
	Replace the old unix_seqpacket and unix_seqpacket_exercise
	tests, which were a little broken and not automatable, with
	unix_seqpacket_test.  It's coverage is a superset of the old
	tests and it uses ATF.  It includes test cases for bugs
	kern/185813 and kern/185812.

r261133
	Fix the Makefiles so that the tests I submitted in r261081 will
	actually get built.

r261615
	tests/sys/Makefile
		use TESTS_SUBDIRS for kern instead of SUBDIRS.  I don't
		think it makes a difference in this case, but
		TESTS_SUBDIRS is generally correct for subdirectories
		that contain tests.