Fix snapshot drag tezt

This MR fixes the *: storage snapshot drag after rolling import tezt.

Indeed, it was working on a flaky behaviour as we were validating blocks, asking the store for caboose/savepoint/checkpoint and validating the data. However, the last store merge was still running when requesting the caboose/savepoint/checkpoint, causing some wrong assumptions. This MR aims to make sure that the store merge is well finished before asking for caboose/savepoint/checkpoint.

The first commit restores the expected behaviour, and thus, this commit alone leads to a test error -- expect.

The second commit fixes the test.

Manually testing the MR

Run dune exec tezt/tests/main.exe -- --title 'Alpha: storage snapshot drag after rolling import' with the first commit -> should fail.

Run the test on the branch -> should work.

Checklist

• [N/A] Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• [N/A] Provide automatic testing (see the testing guide).
• [N/A] 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).
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• Select as Assignee the next person who should take action on that MR