Tests/Pytest: use [Michelson_script] in pytests

Context

Part of #4008 (closed).

Based on !7172 (merged).

Description

As a first step towards merging michelson_test_script and tests_python/contracts_*, this MR adds a mechanism for python tests to lookup contracts versioned using the naming scheme of Michelson_script in Tezt. This works by a small script michelson_script_locator.ml that binds with Michelson_script, and a pytest wrapper around this script in MichelsonScriptLocator. Finally, I update the remaining pytests to lookup scripts through this class. This will make our lives easier in the subsequent steps which are to: 1) merge the tests_python/contracts_* folders into tests_python/contracts and 2) merge tests_python/contracts into michelson_test_scripts.

Manually testing the MR

Read the code, trust the CI.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• 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