Fix getMockForModel() using the incorrect datasource.

Because getMockForModel() does not go through the test datasource
injection in ClassRegistry::init() we need to duplicate the basics of
that logic here. Thankfully, we already have a mock so we can do that
datasource switching without reflection. Of course this means there will
be limitations to how/when this will work, but I feel those scenarios
can probably be solved by not using mocks, or by mocking out the
problematic methods. This set of changes makes getMockForModel() work
with secondary datasources, as one would expect it to do, but I'm not
sure it ever did.

Refs #4694
Fix PHPCS and PHP5.3 error.
Merge pull request #4867 from cakephp/issue-4694

Fix getMockForModel() using the incorrect datasource.