Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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.

MFC 256365
Remove most of the ATF tools and the _atf user.

Approved by: re

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add ATF to the build. This is may be a bit rought around the egdes,
but committing it helps to get everyone on the same page and makes
sure we make progress.

Tinderbox breakages that are the result of this commit are entirely
the committer's fault -- in other words: buildworld testing on amd64
only.

Credits follow:

Submitted by: Garrett Cooper <yanegomi@gmail.com>
Sponsored by: Isilon Systems
Based on work by: keramida@
Thanks to: gnn@, mdf@, mlaier@, sjg@
Special thanks to: keramida@