mailman tests cannot be parallelised (singleton mailman used)

[Robert Collins]

There are two aspects to this:
 - we have just one mailman we import
 - it uses one work area on disk. (/var/tmp/mailman)

The only references to the work area are in:
lib/mailman/Mailman/Defaults.py
VAR_PREFIX = '/var/tmp/mailman'
and
lib/mailman/Mailman/mm_cfg.py
PUBLIC_EXTERNAL_ARCHIVER = '/usr/bin/mhonarc \
-add \
-dbfile /var/tmp/mailman/archives/private/%(listname)s.mbox/mhonarc.db \
-outdir /var/tmp/mailman/mhonarc/%(listname)s \
-definevar ML-NAME=%(listname)s \
-rcfile /var/tmp/mailman/data/lp-mhonarc-common.mrc \
-stderr /var/tmp/mailman/logs/mhonarc \
-stdout /var/tmp/mailman/logs/mhonarc \
-spammode \
-umask 022'

we should be able to:
 - insert a temporary package into Mailman.__path__
 - in that package create a replacement Defaults and mm_cfg.py
 - create a per-testrunner workdir in /tmp

[Robert Collins (lifeless)]

I think I can just change monkey_patch to write a new file and add it to the path.\o/

[Robert Collins (lifeless)]

We can delete the existing files - make them .in files so they can't be accidentally imported, and then the fixture will do all the hard work.

[Robert Collins (lifeless)]

Fixtures 0.3.6 supports the path manipulation we'll need.