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/scripts/invoke-script.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 
+INVOKE_LOG=`mktemp -p $TEST_FOLDER invoke.XXXXX`
+
 pushd $POLKADOT_SDK_FOLDER/cumulus/scripts
-./bridges_rococo_westend.sh $1
+./bridges_rococo_westend.sh $1 >$INVOKE_LOG 2>&1
 popd

bridges/zombienet/scripts/start-relayer.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+RELAY_LOG=`mktemp -p $TEST_FOLDER relay.XXXXX`
+
+pushd $POLKADOT_SDK_FOLDER/cumulus/scripts
+./bridges_rococo_westend.sh run-relay >$RELAY_LOG 2>&1&
+popd