Zope.testrunner is used to run ZODB tests. It was specified as explicit
dependency in buildout.cfg.  However if in SlapOS we install just
ZODB[test] zope.testrunner won't be installed.

-> Fix is by adding explicit zope.testrunner dependency into setup, so that
ZODB[test] installs it. This is the same as what ZODB5 does:

https://github.com/zopefoundation/ZODB/blob/5.6.0-14-g0eae10cd0/setup.py#L48