test: poll for port release in TcpServerTest to fix flake (#166) #443

Job	Run time
test-packaging	4m 55s
test-platforms-and-versions (ubuntu-latest, 8)	3m 40s
test-platforms-and-versions (ubuntu-latest, 17)	3m 46s
test-platforms-and-versions (ubuntu-latest, 11)	3m 55s
test-contract-tests	4m 21s
test-platforms-and-versions (ubuntu-latest, 19)	4m 9s
	24m 46s

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