tests/test-issue433

run-tests.py: avoid using popen2.Popen4 - use subprocess instead

Use subprocess to emulate Popen4 if available - similar to how it is done in
util.py.

Using popen2 under python 2.6 gives
DeprecationWarning: The popen2 module is deprecated.  Use the subprocess module.

#!/bin/sh
# http://www.selenic.com/mercurial/bts/issue433

hg init a
cd a
echo a > a
hg commit -Ama

hg parents -r 0 doesnotexist
true