Improve and expand the IndexSet tests
Improve and expand the IndexSet tests

This gets index_set.cpp back up to 100% line coverage from just the
targeted unit tests.

realm/realm-object-store,realm/realm-js,realm/realm-core,realm/realm-core,realm/realm-js,realm/realm-js,realm/realm-object-store,realm/realm-js,realm/realm-js,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-js,realm/realm-core,realm/realm-js,realm/realm-core,realm/realm-js,realm/realm-js,realm/realm-core