BuildStep to run unit tests from installed packages instead of source tree

[zooko]

I probably should have opened this ticket on  http://buildbot.net instead of  http://allmydata.org in the first place:

 allmydata.org #348 (BuildBot step to run tests from package)

[zooko]

In order to avoid changing the host's system or requiring a VM, do this by installing into a local, newly created subdirectory, and then test the install by adding that directory to the PYTHONPATH.

[dustin]

This sounds like a pretty purpose-specific buildstep. In fact, I would probably implement it with a ShellCommand running a short shell script.