Added Unit-test for get_subtable_size(), with out-commented part that shows how it fails when first column of sub-table is a composite of multiple columns (like mixed and longstrings).
Added Unit-test for get_subtable_size(), with out-commented part that shows how it fails when first column of sub-table is a composite of multiple columns (like mixed and longstrings).

realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core