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.

[jmuchemb]

Currently, 4 failures:

Error in test checkExplicitTransactionManager (ZEO.tests.testZEO.FileStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "ZODB/tests/testZODB.py", line 174, in checkExplicitTransactionManager
    self.assertEqual(r2['item'], 1)
  File "/usr/lib/python2.7/UserDict.py", line 40, in __getitem__
    raise KeyError(key)
KeyError: 'item'

Failure in test checkMultipleUndoInOneTransaction (ZEO.tests.testZEO.FileStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "ZODB/tests/testZODB.py", line 464, in checkMultipleUndoInOneTransaction
    self.assertEqual(root['state'], 0) 
  File "/usr/lib/python2.7/unittest/case.py", line 513, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib/python2.7/unittest/case.py", line 506, in _baseAssertEqual
    raise self.failureException(msg)   
AssertionError: 5 != 0

Failure in test checkResetCache (ZEO.tests.testZEO.FileStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "ZODB/tests/testZODB.py", line 134, in checkResetCache
    self.assertTrue(len(conn._cache) > 0)  # Precondition
  File "/usr/lib/python2.7/unittest/case.py", line 422, in assertTrue
    raise self.failureException(msg)   
AssertionError: False is not true

Failure in test checkResetCachesAPI (ZEO.tests.testZEO.FileStorageTests)
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "ZODB/tests/testZODB.py", line 144, in checkResetCachesAPI
    self.assertTrue(len(conn._cache) > 0)  # Precondition
  File "/usr/lib/python2.7/unittest/case.py", line 422, in assertTrue
    raise self.failureException(msg)
AssertionError: False is not true