[patrickswedish]

Fixes #638.

Summary

mocker.patch.multiple() previously registered the returned dictionary as a single entry in _mock_cache. As a result:

1. The PytestMockWarning context-manager hook was checked on the dictionary rather than on each patched mock.
2. mocker.stop() could not stop an individual mock returned by (or passed to) mocker.patch.multiple().

Changes

• In MockerFixture._Patcher.multiple(), split patcher and patcher.additional_patchers into individual single-attribute patchers (p.attribute_name = None, patcher.additional_patchers.clear()) and start each one through self._start_patch(...).
• If starting any attribute patcher fails midway through multiple(), stop and unregister the previously started patchers from that call before re-raising.
• Remove the MockCacheItem.reset_mocks indirection, since each patched value from patch.multiple is now directly registered in _mock_cache like patch and patch.object.
• Add regression tests in TestStop and TestContextManagerWarnings.