gimarshallingtests: Add test for value array of boxed structs

We already have tests for "array of simple structs"
(gi_marshalling_tests_array_simple_struct_in) and "array of pointers to
boxed structs" (gi_marshalling_tests_array_struct_in), but were missing
"array of boxed structs", which reproduces
https://bugzilla.gnome.org/show_bug.cgi?id=656312