Optimize storage snapshot tezt

This is a followup MR of !9656 (merged) that aims to improve a bit more the memory consumption of this test that was still too greedy. It allows to gain an additional ~300mb of ram and speed up the test a bit (from 1min09 to ~1min).

Additionally, it improves a bit the test's logs

Manually testing the MR

Here is how I've measured the ram consumption. Not sure it actually reflects the exact usage, but it seems decent enough.

• run the test time dune exec tezt/tests/main.exe -- --match 'Alpha: storage snapshot export and import'
• run the magic command while true; do ps -o rss -C octez-node | awk '{rss += $1} END {print rss}' &>> rss.log; sleep 0.1;
• when the test is done kill the magic command
• get the highest ram consumption during the test with cat rss.log| sort -n | tail -n 1

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