Scoru,Test: test roundtripness of encodings

Context

First step in order to close #3230.

• Introduce the new test file in test/pbt/
• Write roundtripness tests for commitments

Manually testing the MR

The CI should execute these tests, you can anyway check that the execution time is not horrible. For this specific MR it's really quick but QCheck2 can create really slow generations.

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• n/a For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR