Tezt/bootstrap: fix test falkiness

This MR aims to fix a flakiness of the Tezt bootstrap test that was not yet discovered as flaky! Indeed, it relies on a value of preservation/preserved cycles of 2 whereas it is now 1 in Alpha and Paris. This may cause a flakiness if the store merge is slower than baking a cycle of blocks.

Manually testing the MR

Green 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