Proto,tests: Fix flakiness of frozen bonds PBT

Context

Frozen bonds PBT have demonstrated to be flaky, mostly because the consistency of the generated scenarios is not checked (leading to impossible situations where a test attempts to transfer more than what a contracts own.

We change the generator to prevent generating inconsistent scenarios. On my machine, I was able to run the test more than 10,000 times without any failure.

Manually testing the MR

dune exec ./tezt/tests/main.exe -- --file 'protocol > integration' --test "016-PtMumbai: frozen bonds" --loop

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