testUncleanShutdownActuallyShutsDown crashes on mac

[artemredkin]

Test Case '-[AsyncHTTPClientTests.HTTPClientTests testUncleanShutdownActuallyShutsDown]' started.
Assertion failed: file /Users/artemredkin/Projects/async-http-client/Sources/AsyncHTTPClient/HTTPClient.swift, line 117
2020-06-15 15:12:01.705061+0100 xctest[27978:8498246] Assertion failed: file /Users/artemredkin/Projects/async-http-client/Sources/AsyncHTTPClient/HTTPClient.swift, line 117

[weissi]

Sorry, forward duping to #263 because it has more info.