Tests/Pytest: rename contracts non_regression/{bug_XXX => XXX_bug}.tz

Context

Part of #4008 (closed).

Description

Here XXX denotes an issue number and not a protocol number. This is inconsistent with the upcoming restructuring of #4008 (closed) and the script versioning mechanism used by Tezt in the Michelson_script module.

Manually testing the MR

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