test: poll for port release in TcpServerTest to fix flake #431

Job	Run time
test-packaging	5m 1s
test-contract-tests	4m 21s
test-platforms-and-versions (ubuntu-latest, 17)	3m 49s
test-platforms-and-versions (ubuntu-latest, 19)	3m 34s
test-platforms-and-versions (ubuntu-latest, 8)	3m 37s
test-platforms-and-versions (ubuntu-latest, 11)	3m 54s
	24m 16s

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