scripts/bw-compatibility-test: add backwards compat test

In this commit, a new backwards compatibility test is added. See the
added README.md file in this commit for all the info.

scripts/bw-compatibility-test/test.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+
+# Stop the script if an error is returned by any step.
+set -e
+
+# DIR is set to the directory of this script.
+DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+source "$DIR/compose.sh"
+source "$DIR/network.sh"
+source "$DIR/.env"
+
+cd $DIR
+
+# Spin up the network in detached mode.
+compose_up
+
+# Ensure that the cluster is shut down when the script exits
+# regardless of success
+trap compose_down EXIT
+
+# Set up the network.
+setup_network
+
+# Print the initial version of each node.
+do_for print_version alice bob charlie dave
+
+# Test that Bob can send a multi-hop payment.
+send_payment bob dave
+
+# Test that Bob can receive a multi-hop payment.
+send_payment dave bob
+
+# Test that Bob can route a payment.
+send_payment alice dave
+
+# Upgrade the compose variables so that the Bob configuration
+# is swapped out for the PR version.
+upgrade_bob
+
+# Wait for Bob to start.
+wait_for_node bob
+wait_for_active_chans bob 2
+
+# Show that Bob is now running the current branch.
+do_for print_version bob
+
+# Repeat the basic tests.
+send_payment bob dave
+send_payment dave bob
+send_payment alice dave
+
+echo "🛡️⚔️🫡 Backwards compatibility test passed! 🫡⚔️🛡️"