Use more stubs

Make use of stubs for ZODB operations to increase testing efficiency.
Introduce db_from_uri argument in the zodb_tween_factory to inject the
stubs.

Add separate test method for get_zodb_root.
Remove make_body_seekable, it's not used by this module
Move import to test method

The test test_db_from_uri is the only test that uses MappingStorage.