Improve dummy implementation

[tzdybal]

Currently dummy implementation of Execution API is basically a noop.
We need to add some logic to be able to create actual "acceptance" style tests.

Also, to reduce confusion, type should be called DummyExecutor (see also: #22).

Also, ExecuteTxs and GetTxs are working completely wrong.

[github-actions]

🎉 This issue has been resolved in version 0.2.1 🎉

The release is available on GitHub release

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