sequencer: test the intialisation of the sequencer data dir without starting a sequencer first.

The following discussion from !11647 (merged) should be addressed:

• @lthms started a discussion: (+1 comment)

  I would rather setup a proxy node instead of a sequencer node here. Then bake a few blocks, then start a sequencer node from the rollup node data-dir, and check that it returns the same block number.