all: fix goroutine leaks in unit tests by adding 1-elem channel buffer (#20666)

This fixes a bunch of cases where a timeout in the test would leak
a goroutine.

p2p/server_test.go

@@ -550,7 +550,7 @@ func TestServerInboundThrottle(t *testing.T) {
 	conn.Close()
 
 	// Dial again. This time the server should close the connection immediately.
-	connClosed := make(chan struct{})
+	connClosed := make(chan struct{}, 1)
 	conn, err = net.DialTimeout("tcp", srv.ListenAddr, timeout)
 	if err != nil {
 		t.Fatalf("could not dial: %v", err)