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/only-mandatory-headers-synced-when-idle.js

@@ -0,0 +1,44 @@
+const utils = require("./utils");
+
+async function run(nodeName, networkInfo, args) {
+    const {wsUri, userDefinedTypes} = networkInfo.nodesByName[nodeName];
+    const api = await zombie.connect(wsUri, userDefinedTypes);
+
+    // parse arguments
+    const exitAfterSeconds = Number(args[0]);
+    const bridgedChain = require("./chains/" + args[1]);
+
+    // start listening to new blocks
+    let totalGrandpaHeaders = 0;
+    let totalParachainHeaders = 0;
+    api.rpc.chain.subscribeNewHeads(async function (header) {
+        const apiAtParent = await api.at(header.parentHash);
+        const apiAtCurrent = await api.at(header.hash);
+        const currentEvents = await apiAtCurrent.query.system.events();
+
+        totalGrandpaHeaders += await utils.ensureOnlyMandatoryGrandpaHeadersImported(
+            bridgedChain,
+            apiAtParent,
+            apiAtCurrent,
+            currentEvents,
+        );
+        totalParachainHeaders += await utils.ensureOnlyInitialParachainHeaderImported(
+            bridgedChain,
+            apiAtParent,
+            apiAtCurrent,
+            currentEvents,
+        );
+    });
+
+    // wait given time
+    await new Promise(resolve => setTimeout(resolve, exitAfterSeconds * 1000));
+    // if we haven't seen any new GRANDPA or parachain headers => fail
+    if (totalGrandpaHeaders == 0) {
+        throw new Error("No bridged relay chain headers imported");
+    }
+    if (totalParachainHeaders == 0) {
+        throw new Error("No bridged parachain headers imported");
+    }
+}
+
+module.exports = { run }