test: poll for port release in TcpServerTest to fix flake #427

Job	Run time
test-platforms-and-versions (ubuntu-latest, 19)	3m 54s
test-packaging	4m 55s
test-platforms-and-versions (ubuntu-latest, 17)	3m 58s
test-platforms-and-versions (ubuntu-latest, 11)	3m 40s
test-contract-tests	4m 28s
test-platforms-and-versions (ubuntu-latest, 8)	3m 36s
	24m 31s

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