Most of Python packages in the Zope world use Buildout:

svn co svn+ssh:// plone.z3cform
cd plone.z3cform
bin/test -pvc

Now suppose you want to change the buildout environment somehow, e.g. use the current development version of zope.testing instead of whatever is specified in buildout.cfg. Don't edit the existing buildout.cfg (you might accidentally commit your local debug changes), instead create a new cfg file, e.g. test.cfg:

extends = buildout.cfg
develop += ../zope.testing

# override any existing version pins
zope.testing =

Now re-run buildout

bin/buildout -c test.cfg
bin/test -pvc

And the tests should be run with the newest zope.testing.code.

Only this does not work with plone.z3cform, and I have no clue why. It generally works with other packages (at least those that use the zc.recipe.testrunner rather than collective.recipe.z2testrunner). Buildout is like that sometimes :(