gimarshallingtests: Add test helpers for marshaling of object arguments

gimarshallingtests: Add test helpers for marshaling of object arguments

Add a number of vfuncs and methods which can be used for testing
marshaling of objects with different combinations of ownership
transference. An important part of these test vfuncs and methods is
they do not pass object returns and arguments through them. Instead
the methods return reference counts and floating attributes. This allows
isolation and ensures any problem with round trip object marshaling
does not obscure what should be tested from the perspective of C as the
caller of a vfunc. Tests and vfuncs can then be written in any language
with gi bindings.

https://bugzilla.gnome.org/show_bug.cgi?id=687522