Use mock stack in callbacks ibc_middleware_test.go

[colin-axner]

Summary

Use the mock stack in callbacks ibc_middleware_test.go instead of the transfer stack. This allows for more predictable maintenance of the unit tests. Currently it isn't possible as the mock would need to define its own packet data type which implements the GetSender function. When that functionality is added, the tests can be rewritten to use the mock stack instead of transfer, thus simplifying logic

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