BUG Revert some changes to ManyManyList

This fix reverts some changes made in the ORM rewrite of 3.2 to ManyManyList, as well as fixes some issue raised as the result of some behavioural changes in testing. Notes will follow on the individual line items.
This is necessary due to `ClassInfo::dataClassesFor` returning classes which should have a table, but were not scaffolded for the current test (i.e. existed in the SS_ClassManifest, but did were not specified in the $extraDataObjects variable for the currently executing test).