Fix make distclean Makefile.am removal

The file tests/zfs-tests/tests/stress/Makefile.am gets mistakenly
removed by the distclean target because it's empty. Adding a
SUBDIRS = line prevents the removal.

This directory is being preserved as the location to add assorted
stress tests. These may include but are not limited to.

  http://kernel.ubuntu.com/~cking/stress-ng/
  https://github.com/zfsonlinux/zfsstress/

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5308