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)

Plug the ATF tests into the build.

This is a MFC into stable/10 of:
- r257849 Add libatf-c++ to the prebuild libs.
- r257853 Build and install the atf tests.
- r258233 Move all atf directories to the tests mtree.
- r258285 Fix the build of some ATF tests.

This change is "make tinderbox" clean on ref10-amd64 with the default
settings of WITHOUT_TESTS. It is likely for the WITH_TESTS build to
still be broken because not all relevant changes have been merged yet.

Build and install the atf tests.

Reviewed by: freebsd-testing
Approved by: rpaulo (mentor)