WIP: tests: run generic ZODBTests from ZODB

[jmuchemb]

testZODB from ZODB is another set of generic tests that can be reused to check storage implementations. NEO runs them.

Note sure if ZODBTests is pertinent for any storage behind ZEO so I suggest to only add it to FileStorageTests. And contrary to other tests, ZODBTests expects a DB object as a _db attribute of the test case, hence the hacky property.

More generally, it would be good to review the ZODB test suite so that at least, storage implementations don't miss generic tests.