rcmgr: flaky resource manager TestEcho

[marten-seemann]

=== RUN   TestEcho
      echo_test.go:68: 
          	Error Trace:	echo_test.go:53
          	            				echo_test.go:68
          	Error:      	Not equal: 
          	            	expected: itest.EchoStatus{StreamsIn:1, EchosIn:1, EchosOut:1, IOErrors:0, ResourceServiceErrors:0, ResourceReservationErrors:0}
          	            	actual  : itest.EchoStatus{StreamsIn:1, EchosIn:1, EchosOut:0, IOErrors:0, ResourceServiceErrors:0, ResourceReservationErrors:0}
          	            	
          	            	Diff:
          	            	--- Expected
          	            	+++ Actual
          	            	@@ -3,3 +3,3 @@
          	            	  EchosIn: (int) 1,
          	            	- EchosOut: (int) 1,
          	            	+ EchosOut: (int) 0,
          	            	  IOErrors: (int) 0,
          	Test:       	TestEcho
  --- FAIL: TestEcho (3.[97](https://github.com/libp2p/go-libp2p/runs/6048928426?check_suite_focus=true#step:9:97)s)