ENH: speed up testing when --processes arg is given to test_fast.sh
I know this is a stupid one liner but...this allows the pytables fixtures to be shared, but not split (because PyTables is not reentrant [only reading is thread-safe]), during a test run with multiple processors. I get a 3x speed up (75 secs to 25 secs) just by adding this single class variable.

Whoa travis is failing to install pandas...
 @y-p I see you are getting similar errors on your travis build as well.
 nope, all sweetness and light there. are you updated to latest master? there was a glitch
with a recent PR that touched the travis scripts.
 Yep I am and I was when I pushed that...weird. [This](http://travis-ci.org/cpcloud/pandas/builds/6459979) shows that one of your builds failed. I think I can see it because I'm still tracking your fork.
 checking.
 The log looks like a pypi hiccup which failed some dep installs including cython which caused it
to crash and burn.

Looks ok now:
https://travis-ci.org/y-p/pandas/jobs/6460587
 I was actually hoping that waiting on IO would make parallelism worthwhile,
to speed up travis with one thread. But i see no noticeable change.
 Should I close this? Seems not worthwhile anymore.
 I'm more then glad to make things faster if it's possible, but I just can't repro on
my local machine OR travis, so if this isn't faster in practice - yep, close.
