Relocate subversion path setting in svnadmin_available() to settings.py

When reviewing the testrunner code, I noticed that the SVNADMIN_PATH was being set in `mysite.base.depends.py`. It really should be set in the `settings.py` file. 

I also added a section to the advanced testing documentation that points out that the subversion mission tests depend on svn and svnadmin being installed to run.
so much +1