test-unit-serialize: add a very basic test that command deserialization works
    
    We should test both serialization and deserialization works properly.
    But the serialization/deserialization code is deeply entwined with the
    manager state, and I think quite a bit of refactoring will be required before
    this is possible. But let's at least add this simple test for now.
    
    (cherry picked from commit 35243b77360c9cc7d1446617fe4fd304bfdecd4c)