In steps.source.cvs: pass self.timeout to 'rmdir'

Hi,

I know, this is quite horrible, but we do have a large codebase and AIX and Solaris 10 are pretty slow when it comes to removing directories.

Can we pass the CVS step's `timeout` to `rmdir`?

This pull request has the side-effect that the `rmdir` commands now have a default timeout of 1200s (instead of their slave-side default 120s).

Additionally, I'm not really proud of what I did to the tests. Can I somehow avoid hard-coding the BuildStep's default timeout inside the test code? Should I test for `step.timeout`?