[mono] Enable UnmanagedCallersOnlyTest tests for the supported features in Mono AOT

[ivanpovazan]

Enable UnmanagedCallersOnlyTest tests for Mono AOT, based on the support for the attribute introduced here #69759

This requires conditional exclusion of the IL portion of the tests and their references from C#.

Related to #71532

[ivanpovazan]

Closing this issue since we implemented a different approach and did not exclude tests which were testing support for this attribute on Mono