Fix tests on Python 2

[cjwatson]

ZODB 5.5.0 added an "import mock" to ZODB.tests.util (or "from unittest
import mock" on Python 3, which is in the standard library). ZODB[test]
included the new dependency, but zope.component wasn't using that for
tests.

However, ZODB.tests.util.DB was just an import of
ZODB.MappingStorage.DB, so we can just as easily use the latter and
avoid the extra dependency.

[jamadden]

Can you please mention where this was a problem? I must be missing something, but I'm not sure what.

The last Python 2.7 build on master was from about 14 hours ago as I write this, and it passed. It uses 'pip install -U -e .[test]' to get ZODB 5.5.1 and then uses Sphinx to run doctests which appears to include docs/api/persistent.rst. I can't find any indication that it installed mock.

[cjwatson]

I'm somewhat confused that it passed on Travis. I can't look properly right now, but I just ran tox locally in the ordinary way and the api/persistent test failed there.

[mgedmin]

LGTM.

I can reproduce the failure with tox -e py27. I guess the Travis images come with mock preinstalled for some reason.

[jugmac00]

I guess the Travis images come with mock preinstalled for some reason.

At least it was (documented) for Trusty:
https://docs.travis-ci.com/user/reference/trusty/#pre-installed-pip-packages