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).

tests/numeric.at

@@ -27,14 +27,17 @@ tar $1 -vvf a dir |dnl
 ])
 
 AT_TAR_CHECK([
-mkdir dir
-genfile --file dir/file
-
 MYUID=$(id -u) || AT_SKIP_TEST
 MYGID=$(id -g) || AT_SKIP_TEST
 MYUSR=$(id -un) || AT_SKIP_TEST
 MYGRP=$(id -gn) || AT_SKIP_TEST
 
+mkdir dir
+# Ensure correct group id on BSDs.
+chown :$MYGID dir >/dev/null 2>/dev/null
+genfile --file dir/file
+
+
 TESTOP([--create])
 TESTOP([--list])
 TESTOP([--diff])