ssh: drop extra connection Close call in TestMuxReadWrite

The extra Close call runs in a separate goroutine, and races with the
deferred Close call, leading to test flakes if the extra call fails

Change-Id: Ia1a2fb3932906da7cd0a8854b55f82785bb33716
Reviewed-on: https://go-review.googlesource.com/104335
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/mux_test.go

@@ -108,10 +108,6 @@ func TestMuxReadWrite(t *testing.T) {
 		if err != nil {
 			t.Fatalf("Write: %v", err)
 		}
-		err = s.Close()
-		if err != nil {
-			t.Fatalf("Close: %v", err)
-		}
 	}()
 
 	var buf [1024]byte