flaky TestOpenStreamSyncShutdown

[marten-seemann]

https://github.com/quic-go/webtransport-go/actions/runs/4070366077/jobs/7011132327

=== RUN   TestOpenStreamSyncShutdown/bidirectional_streams
    webtransport_test.go:449: 
        	Error Trace:	/home/runner/work/webtransport-go/webtransport-go/webtransport_test.go:437
        	            				/home/runner/work/webtransport-go/webtransport-go/webtransport_test.go:449
        	Error:      	Should be in error chain:
        	            	expected: %!q(**webtransport.ConnectionError=0x9412f98)
        	            	in chain: 
        	Test:       	TestOpenStreamSyncShutdown/bidirectional_streams