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.
