Tezt: fix DAC flaky tests

Fixes a flaky DAC test: Oxford: Testing Full DAC infrastructure (test DAC disconnects from L1). This test was reported as flaky during the last few weeks, see https://grafana.nomadic-labs.cloud/d/tezos_flaky_tests/tezos-flaky-tests?orgId=1&var-job_user=nomadic-margebot&from=now-30d&to=now

It seems that it was even more flaky in the context of %Non-Blocking RPCs

To fix the issue, it simply waits for the DAC nodes to be synchronized to the L1. Not being in sync was resulting to a stuck state with unreachable RPCs.

Manually testing the MR

Green CI

Checklist

• 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)
• 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).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR