Tezt/tezos_lib: bake_for provides a "pending_operations" mempool file

Context

This MR attempts to fix flaky tests in tezt.

Fixes #3097 (closed).

It's an alternative to !5339 (closed)

Manually testing the MR

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