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

Set up the /usr/tests hierarchy.

This is a MFC of the following into stable/10:
- r257097 Set up the /usr/tests hierarchy.
- r257098 Add missing WITHOUTTESTS file.
- r257100 Add a tests(7) manual page.
- r257105 Disable WITHTESTS= for now.
- r257848 Fix buildworld when WITHTESTS is enabled.
- r257850 Subsume the functionality of MKATF into MKTESTS.
- r257851 Handle the removal of the test suite when WITHOUTTESTS=yes.
- r257852 Install category Kyuafiles from their category directories.
- r258232 Install BSD.tests.mtree when MKTESTS is yes.

Note that building with WITH_TESTS is still broken at this point (and
hence why WITHOUT_TESTS is the set as the default). Subsequent pullups
will fix the remaining issues.

Install category Kyuafiles from their category directories.

Move the installation of /usr/tests/lib/Kyuafile from src/tests/lib/
to src/lib/. This is to keep the src/tests/ hierarchy unaware of the
rest of the tree, which makes things clearer in general. In particular:

1) Everything related to the construction of /usr/tests/lib/ is kept
in src/lib/. There is no need to think about different directories
and how they relate to each other. (The same applies for libexec,
usr.bin, etc. but these are not yet handled.)

2) src/tests becomes the place to keep cross-functional test programs
and nothing else, which also helps in simplifying things.

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