Allow mixing bsd.files.mk with bsd.subdir.mk.

If a single Makefile wants to recurse into subdirectories and also
wants to install files, bsd.files.mk's targets would get ignored in
favor of those defined by bsd.subdir.mk because installfiles would
not get defined in bsd.files.mk.

Prevent this from happening by defining the targets in bsd.files.mk
with auxiliary names and listing them as dependencies of installfiles
instead.

This is required by bsd.test.mk, which needs to install a Kyuafile
in pretty much all cases but may also need to recurse into
subdirectories for build purposes.

Submitted by:	Julio Merino jmmv google.com
Reviewed by:	sjg
MFC after:	2 weeks
Move the TESTSBASE definition to bsd.own.mk.

We need to be able to reference the value of TESTSBASE without requiring
the inclusion of bsd.test.mk (e.g. in etc/Makefile), so move its definition
to the more generic bsd.own.mk.

Submitted by:	Julio Merino jmmv google.com
Reviewed by:	sjg
MFC after:	2 weeks
Set up the /usr/tests hierarchy.

Populate /usr/tests with the only test programs that currently live
in the tree (those in lib/libcrypt/tests/) and add all the build
machinery to accompany this change.

In particular:

- Add a WITHOUT_TESTS variable that users can define to request that
  no tests be put in /usr/tests.
- Add a top-level Kyuafile for /usr/tests and a way to create similar
  Kyuafiles in top-level subdirectories.
- Add a BSD.tests.dist file to define the directory layout of
  /usr/tests.

Submitted by:	Julio Merino jmmv google.com
Reviewed by:	sjg
MFC after:	2 weeks
Add missing WITHOUT_TESTS file.

Submitted by:	Julio Merio jmmv google.com
Reviewed by:	sjg
MFC after:	2 weeks
Add missing plain.test.mk.

Submitted by:	Julio Merino jmmv google.com
MFC after:	2 weeks
Add a tests(7) manual page.

This manual page intends to describe the structure and behavior of
the FreeBSD test suite installed in /usr/tests.  The contents have
been inherited from the NetBSD manual page.

As a side effect, this also updates the hier(7) manual page to
mention /usr/tests and points at tests(7) for more details.

Submitted by:	Julio Merino jmmv google.com
Reviewed by:	sjg
MFC after:	2 weeks