cmd/geth: fix port clash in genesis test

cmd/geth/genesis_test.go

@@ -100,7 +100,10 @@ func TestCustomGenesis(t *testing.T) {
 		runGeth(t, "--datadir", datadir, "init", json).cmd.Wait()
 
 		// Query the custom genesis block
-		geth := runGeth(t, "--datadir", datadir, "--maxpeers", "0", "--nodiscover", "--nat", "none", "--ipcdisable", "--exec", tt.query, "console")
+		geth := runGeth(t,
+			"--datadir", datadir, "--maxpeers", "0", "--port", "0",
+			"--nodiscover", "--nat", "none", "--ipcdisable",
+			"--exec", tt.query, "console")
 		geth.expectRegexp(tt.result)
 		geth.expectExit()
 	}