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/tests/0002-mandatory-headers-synced-while-idle-westend-to-rococo.zndsl

@@ -0,0 +1,26 @@
+Description: While relayer is idle, we only sync mandatory Westend (and a single Westend BH) headers to Rococo BH.
+Network: ../../../cumulus/zombienet/bridge-hubs/bridge_hub_rococo_local_network.toml
+Creds: config
+
+# step 1: initialize Rococo bridge hub
+bridge-hub-rococo-collator1: run ../scripts/invoke-script.sh with "init-bridge-hub-rococo-local" within 60 seconds
+
+# step 2: sleep some time before starting relayer. We want to sleep for at least 1 session, which is expected to
+# be 60 seconds for test environment.
+sleep 120 seconds
+
+# step 3: start relayer
+bridge-hub-rococo-collator1: run ../scripts/start-relayer.sh within 60 seconds
+
+# it also takes some time for relayer to initialize bridge, so let's sleep for 5 minutes to be sure that parachain
+# header has been synced
+
+# step 4: ensure that relayer is only syncing mandatory headers while idle. This includes both headers that were
+# born while relay was offline and those in the next 100 seconds while script is active.
+bridge-hub-rococo-collator1: js-script ../helpers/only-mandatory-headers-synced-when-idle.js with "300,westend-at-rococo" within 600 seconds
+
+# wait until other network test has completed OR exit with an error too
+asset-hub-rococo-collator1: run ../scripts/sync-exit.sh within 600 seconds
+
+# wait until other network test has completed OR exit with an error too
+asset-hub-rococo-collator1: run ../scripts/sync-exit.sh within 600 seconds