Merge manyprocesses-3404-5: run fewer processes in testManyProcesses

Author: glyph

Reviewer: jml

Fixes: #3404

This adjusts the number of concurrent processes run by testManyProcesses
to stay under some obscure, undocumented limit on windows XP and
facilitate more reliable build results.  It should also make the tests
slightly faster on other platforms, so we're changing it globally rather
than platform-specifically.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43371 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb