TestClient does not always terminate its server

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TestClient integration test fails despite success: https://travis-ci.com/CAU-Kiel-Tech-Inf/socha/builds/143036739

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https://travis-ci.com/github/CAU-Kiel-Tech-Inf/socha/builds/155503597

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test-client.log
Weird thing in this case: There were no client logs, and the test-client even kept on running, but without producing any logs.

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Also: TestClient/its server often continues running after a test, impeding successive tests