tests: fix numeric.at for BSD

While creating file, BSD kernels inherit the group ownership from
parent directory.
http://lists.gnu.org/archive/html/bug-tar/2016-06/msg00000.html

* tests/numeric.at: Attempt to 'chown' the newly created directory
to proper group (at least on affected machines that command is
expected to succeed).

THANKS

@@ -95,6 +95,7 @@ Christian Laubscher	christian.laubscher@tiscalinet.ch
 Christian T. Dum	ctd@mpe-garching.mpg.de
 Christian von Roques	roques@pond.sub.org
 Christian Wetzel	wetzel@phoenix-pacs.de
+Christian Weisgerber	naddy@mips.inka.de
 Christoph Litauer	litauer@mailhost.uni-koblenz.de
 Christophe Colle	colle@krtkg1.rug.ac.be
 Christophe Kalt		Christophe.Kalt@kbcfp.com