Add host.test.mk for DIRDEPS_BUILD

If we are doing DIRDEPS_BUILD and MK_RUN_TESTS is yes
we can hook tests associated with dirs to be built into the graph
such that we avoid circular dependencies but the build will fail
should any test fail.

Make MK_RUN_TESTS independent of MK_TESTS
also for non-DIRDEPS_BUILD ensure we have RELDIR and check it
against patterns in HOST_RUN_TESTS.

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D50387