python version-specific codepaths in tests

[Gordon Tyler]

John Arbash Meinel wrote:
>> So being able to test that "bzr: interrupted" is output to stderr on
>> SIGINT is proof that things were cleaned up?
> 
> Well, testing that it doesn't dump a traceback to stderr is :)

Just did a little research on signals handling in python. SIGTERM is 
apparently not as clean an exit as we may want. I'll investigate 
changing it back to SIGINT with the workaround for win32.

Ciao,
Gordon