Reviewing the test code, the code states "set the window size to 0 to prevent the stream from being closed" here

but the rfc states, "Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow-control window."

in section 6.9.1

So, if the response to the request contains no data, then this test will fail, because the server can process the request and close the stream, reducing the number of streams in use - so the test will never fail as the maximum will never be exceeded - it will hang.