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.
% fail
could not talk to new inotify server: No such file or directory
abort: could not start server: File exists
% inserve
% status
? hg.pid