python: add a failed test to show Popen deadlock

python: add a failed test to show Popen deadlock

`Popen.wait()` will deadlock when using stdout=PIPE and/or stderr=PIPE and the
child process generates large output to a pipe such that it blocks waiting for
the OS pipe buffer to accept more data. Use communicate() to avoid that.

This patch is commited to show the issue, a fix patch will come later.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>