gimarshallingtests: Add a marshalling test case for GPtrArrays and GArrays of structures
Prior to this, the only marshalling test cases available for GPtrArrays were for GPtrArrays of strings.
This commit adds a marshalling test case for GPtrArrays of structures, with the same objective than the gi_marshalling_tests_array_zero_terminated_return_struct() test case.
This also adds a similar marshalling test case for GArrays of structures, for completeness.
This is a follow-up to gjs#9 (closed) where a regression has been detected with these types of GPtrArrays.
These new test cases will be used in gjs!311 (merged) to avoid encountering the same type of regression in the future.