Tests: improve rollup node snapshot tests to remove flakyness

What

This MR refines the tests for the rollup node snapshots to make them less flaky.

Why

The tests were reported as flaky for two reasons:

1. the on-fly-compression equivalence check could fail because one of the export command could fail
2. the "import published commitments snapshot" check could fail if a commitment was made before or a delay in the injection loop meant the commitment was not included before the import.

How

1. Remove on-the-fly compression check for snapshots: this option is rarely used because if favors space at the cost of speed and the code is not tricky.
2. Increase the commitment period for snapshot tests : Having a greater commitment period allows to avoid the case where a commitment is made before a check or a commitment is not published.

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml snapshot -j 4 --loop