Fix and re-enable CliqueProposeRpcAcceptanceTest

[fab-10]

PR description

the flakiness was due to the fact that during cluster start, when there are more validators at genesis, there could be a race condition and more that one validator could create block #1 resulting in a network fork.
The solution the is to always start with only one validator, waiting for the cluster to be in sync and then add other validators.

Fixed Issue(s)

fixes #7973

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• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests

[macfarla]

Love it @fab-10