vala: Don't just guess and check for a matching base_interface_method too

vala: Don't just guess and check for a matching base_interface_method too

This fixes class with multiple interfaces which require implementations of
methods with conflicting naming while the explicit interface-type reference
is not present yet.

Extend the present test-case for runtime checking. This will still silently
connect matching base-class methods as before as introduced in
e1a3ff9470763e7c6ff5a887036390bd418f4e46

Fixes https://gitlab.gnome.org/GNOME/vala/issues/548