Prevent core files from XFAIL test.

* On some machines this test will create core files on every
    make check
  if not disallowed with ulimit -c 0.

tests/test_xmap_all2all_fail_run.in

 #! @SHELL@
 [ x"@MPI_LAUNCH@" != xtrue ] || exit 77
+ulimit -c 0
 @abs_top_builddir@/libtool --mode=execute \
   @MPI_LAUNCH@ -n 1 @abs_builddir@/test_xmap_all2all_fail -s small \
   >/dev/null 2>&1 \