Move ZODB and Zope setup and teardown to the layer setup and teardown

I did a bit of profiling, and contrary to a widely held belief, most of the wallclock time spent in the tests is not spent starting up or closing the browser (WebDriver), but rather at setting up and tearing down the plone site (ZODB, Zope) at every suite setup and suite teardown.

To me this'd seem like a big win to move those setups to the layer setup and layer teardown, as they're shared across all of the suites.

Start by locating the suite setup and teardown and the corresponding layer setup and teardown in the repository. Trace where the ZODB and Zope site are initialized and closed, then run the functional test suite to establish its current behavior. Done means those operations are shared at layer scope rather than repeated for every suite, with the tests still passing.