test-unit-serialize: add a very basic test that command deserialization works

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.