Test/SCORU/Node: ensure correct commitment published in case of reproposal in reorganisation

Context

This MR adds a new test which simulates (artificially) a reorganisation in which a block is reproposed and ensures that the correct commitment is published.

This test is specifically made for the fix of !9652 (merged) (I have tested that it fails with "Safety error ..." without it).

This MR also contains a fix for incorrectly setting the L2 head in case of a reproposal.

Manually testing the MR

dune exec tezt/tests/main.exe -- -f sc_rollup.ml reproposal