Fix buildworld when WITH_TESTS is enabled.

The addition of the TESTS knob and its enabling of the build of tests in
lib/libcrypt/tests/ broke the build.  The reason is that we cannot descend
into tests/ subdirectories until all prerequisites have been built, which
in the case of tests may be "a lot of things" (libatf-c in this case).

Ensure that we do not walk tests/ directories during the bootstrapping of
the libraries as part of buildworld.

Reviewed by:	freebsd-testing
Approved by:	rpaulo (mentor)
Add libatf-c++ to the prebuild libs.

Some tests may require C++ so we must ensure this library exists as part
of the bootstrap process or else they will fail to build.  Do this by
just depending on lib/atf as part of the bootstrap libraries instead of
using lib/atf/libatf-c.

Submitted by:	Garrett Cooper <yaneurabeya at gmail dot com>
Approved by:	rpaulo (mentor)
Subsume the functionality of MK_ATF into MK_TESTS.

There is no reason to keep the two knobs separate: if tests are
enabled, the ATF libraries are required; and if tests are disabled,
the ATF libraries are not necessary.  Keeping the two just serves
to complicate the build.

Reviewed by:	freebsd-testing
Approved by:	rpaulo (mentor)
Handle the removal of the test suite when WITHOUT_TESTS=yes.

Add all files from /usr/tests to the obsoleted files list when the
build of the tests is disabled via the WITHOUT_TESTS knob.  Do this
automatically so that we do not have to suffer the pain of maintaining
such list by hand.

Reviewed by:	freebsd-testing
Approved by:	rpaulo (mentor)
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)
Build and install the atf tests.

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