DMQ integration tests are flaky in CI

[jpraynaud]

Why

We have noticed some flakiness in the integration tests of the DMQ consumer and publishers:

• https://github.com/input-output-hk/mithril/actions/runs/18273656583/job/52020738050#step:5:1280
• https://github.com/input-output-hk/mithril/actions/runs/18273656583/job/52020738158#step:5:2229

What

Investigate and fix the source of flakiness in the DMQ integration tests

How

• Investigate the source of flakiness (due to lack of deterministic timestamp provider in KES signers)
• Fix the source of flakiness