Fix a flaky zombienet test - 0004-coretime-smoke-test (#6236)

In the test log I noticed that the batch transaction which configures
the coretime chain fails. However when rerunning the transaction
manually - it worked. Then I noticed that the coretime chain is
initially registered via zombienet and then re-registered via
`0004-configure-relay.js`. Because of this there is a period of time
when it's not producing blocks and `0004-configure-broker.js` fails to
setup the coretime chain. My theory is that the transaction has failed
because the coretime chain is stalled during the re-registration.

Fixes https://github.com/paritytech/polkadot-sdk/issues/6226