autobuild: Stop waf uninstall from removing test_tmpdir

autobuild: Stop waf uninstall from removing test_tmpdir

Most of the autobuild tasks run "make distcheck", which does a
recursive "waf configure make install uninstall".  "waf uninstall"
(via BuildContext.install() in Build.py) removes empty directories all
the way up the directory tree.  This means that it removes
test_tmpdir, if it is empty, and any empty directories above it.

While this is arguably a waf bug, the simplest solution is to make
test_tmpdir non-empty so it don't get removed.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12703

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Mar 21 10:37:08 CET 2017 on sn-devel-144