When writing tests that involve the usage of an async dbus api, it is non-trivial to extract and inspect the response from the related callback since the object used for the response is passed as a unique_ptr and hence ownership cannot be retained.
A solution was implemented and copied over in other tests here: https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/login_manager/session_manager_impl_unittest.cc#236

Since this is a fairly useful mechanism for testing, it needs to be refactored out and placed in a shared location.