MFC r289172,r290254:

r289172:

Refactor the test/ Makefiles after recent changes to bsd.test.mk (r289158) and
netbsd-tests.test.mk (r289151)

- Eliminate explicit OBJTOP/SRCTOP setting
- Convert all ad hoc NetBSD test integration over to netbsd-tests.test.mk
- Remove unnecessary TESTSDIR setting
- Use SRCTOP where possible for clarity

r290254:

Remove unused variable (SRCDIR)

MFC r288199,r288246:

Add missing CLEANFILES.

MFC r287360:
Fix t_spawnattr test for attributes handling by posix_spawn(3).
Connect it to the build.

MFC r288199,r288246:

Add missing CLEANFILES.

MFC r287360:
Fix t_spawnattr test for attributes handling by posix_spawn(3).
Connect it to the build.