Add two new zombienet tests for bridges (manual run) (#3072)

extracted useful code from #2982 

This PR:
- adds test 2 for Rococo <> Westend bridge: checks that relayer doesn't
submit any extra headers while there are no any messages;
- adds test 3 for Rococo <> Westend bridge: checks that relayer doesn't
submit any extra headers when there are messages;
- fixes most of comments from #2439 (like: log names, ability to run
specify test number when calling `run-tests.sh`).

Right now of all our tests, only test 2 is working (until BHs will be
upgraded to use async backing), so you can test it with
`./bridges/zombienet/run-tests.sh --test 2` locally.

bridges/zombienet/helpers/chains/rococo-at-westend.js

@@ -0,0 +1,6 @@
+module.exports = {
+	grandpaPalletName: "bridgeRococoGrandpa",
+	parachainsPalletName: "bridgeRococoParachains",
+	messagesPalletName: "bridgeRococoMessages",
+    bridgedBridgeHubParaId: 1013,
+}