[Piglit] Python 2.7.4/5 bad file descriptor bug should be fixed soon

[Kenneth Graunke]

A number of people (at least me, cworth, and ajax) have been hitting a
bug where tests randomly fail with a "Bad file descriptor" error.  It
only occurs with Python 2.7.4 and 2.7.5.  Other people have strangely
not been affected.

I did some searching and discovered this Python bug:
http://bugs.python.org/issue18851

Apparently, it looks like a known bug in Python, where if a
subprocess.Popen fails in one thread, it closes the file descriptors out
from under subprocesses in other threads.

A fix landed in Mercurial back in August, so hopefully things will work
with Python 2.7.6.  According to the release schedule
[http://www.python.org/dev/peps/pep-0373/] RC1 is supposed to come out
this Saturday, with the final release on November 2nd.

In the meantime, using Python 2.7.3 works, as does running with the -1
(--no-concurrency) option.

--Ken