`ZEO[test]` should be installed when testing, so that `zope.testing` is
installed.
Otherwise, an import error may be raised when running the test if
`zope.testing` has not been manually installed.

--------
kirr:

Adjust tox.ini to no longer install zope.testing explicitly since we are
now requesting ZEO[test] under our own tests extra, and tox.ini installs
.[test] as the primary package.

/reviewed-by @kirr
/reviewed-on nexedi/wendelin.core!27