cmd/devp2p: add eth protocol test suite (#21598)

This change adds a test framework for the "eth" protocol and some basic
tests. The tests can be run using the './devp2p rlpx eth-test' command.

cmd/devp2p/main.go

@@ -81,7 +81,7 @@ func commandHasFlag(ctx *cli.Context, flag cli.Flag) bool {
 
 // getNodeArg handles the common case of a single node descriptor argument.
 func getNodeArg(ctx *cli.Context) *enode.Node {
-	if ctx.NArg() != 1 {
+	if ctx.NArg() < 1 {
 		exit("missing node as command-line argument")
 	}
 	n, err := parseNode(ctx.Args()[0])