Fix tests broken after changes for PY-4073

* Fix typo in special attribute name
* Split testPropertyOfUnionType in two test cases: for resolved (with type
known) and unresolved attribute since type of slice.start is known
now because of type annotation in regenerated binary skeleton for
MockSdk2.7.
Refactoring of grouping configurables. Use the following registry key to switch on this functionality: ide.file.settings.order.new