Move FreeBSD Test Suite-specific code to a suite.test.mk file.

The new suite.test.mk file contains all the logic needed to install test
programs under /usr/tests/ and to support Kyua as the run-time engine.
This file is included by default by bsd.test.mk so Makefiles do not need
to care about its existence.

Specific Makefiles can define NOT_FOR_TEST_SUITE to indicate that whatever
test programs they are building are not supposed to be installed under
/usr/tests/ nor run by Kyua.  (The effect of passing this setting is that
suite.test.mk is simply not included.)

NOT_FOR_TEST_SUITE should never be used by Makefiles in the base system.
This functionality is provided so that third-parties can hook in their
own test code, with different semantics, if they wish.  This was asked
for by sjg@.
Remove unnecessary svn:executable property from source file.

The atf cp_test.sh sample file should have never been marked executable in
the first place because this file needs to be "built" first before being
usable.