useless /usr/tests tree for MKATF=no builds

[Nicolas Joly <njoly%pasteur.fr@localhost>]

Hi,

While checking MKATF=no builds, i noticed that, in that case, we do
generate an almost useless `/usr/tests' tree.

While previously some shell tests could be run mannually, we can't now
because the interpreter switched to /usr/bin/atf-sh (since ATF 0.9).
Other test that require c/c++ APIs are not even built ... Only remains
a few input/results files.

Unless we plan to integrate stuff that will not use ATF, i'm currently
thinking of making tests set optional.

This will require a new mtree list (etc/mtree/NetBSD.dist.tests), a
new variable `test' in distrib/sets/sets.subr to allow generating the
corresponding `tests.tgz' archive. And maybe some other adjustements
somewhere ...

Before stepping into this one, what do people think ?

Thanks.

-- 
Nicolas Joly

Biological Software and Databanks.
Institut Pasteur, Paris.