Tezt: improve storage_maintenance test to acknowledge context split

This simple MR aims to add an additional event check to make sure that, even if the storage maintenance is delayed, the context split are triggered at the right time.

Indeed, the split function creates a new suffix file, also called "chunk", into the irmin's file hierarchy. To be optimal, the split function is expected to be called directly after committing, to the context, a commit that will be a future candidate of a GC target. If the last commit of a chunk appears not to be the candidate of a future GC, it may result in keeping chunks containing partially needed data. This is not an issue, but it should be avoided to prevent storing unnecessary data and thus, to minimize the disk footprint.

Manually testing the MR

dune exec tezt/tests/main.exe -- --title "Alpha: storage maintenance custom delay" --verbose (or any protocol) + 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