patch 9.0.1880: Vim9: Need more tests for inheritance

patch 9.0.1880: Vim9: Need more tests for inheritance

Problem:  Vim9: Need more tests for inheritance
Solution: Add access tests and fixes.

`inside_class` fix from yegappan. `object_index_from_itf_index` fix
access of member on class extending class implementing interface.

Based on tests from Vim9: Class/Object member variable access control #12979

closes: #13032
related: #12979

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>