Run bridges zombienet tests on CI (#2439)

Brridges zombienet tests are non-standard - zombienet currently missing
multiple relay chains support (see e.g.
https://github.com/paritytech/zombienet/pull/796), so we need to go live
with two relay networks, their parachains + custom test runner (which
e.g. doesn't shutdown net when its tests are finished and instead waits
for both networks tests to complete). So we are stuck with native
zombienet provider => this PR is an attempt to gather everything in a
single docker container and run tests there ~Draft, because it is far
from finishing - what I want now is to see how it works on CI~

cumulus/scripts/bridges_common.sh

@@ -1,27 +1,21 @@
 #!/bin/bash
 
-function ensure_binaries() {
-    if [[ ! -f ~/local_bridge_testing/bin/polkadot ]]; then
-        echo "  Required polkadot binary '~/local_bridge_testing/bin/polkadot' does not exist!"
-        echo "  You need to build it and copy to this location!"
-        echo "  Please, check ./parachains/runtimes/bridge-hubs/README.md (Prepare/Build/Deploy)"
-        exit 1
-    fi
-    if [[ ! -f ~/local_bridge_testing/bin/polkadot-parachain ]]; then
-        echo "  Required polkadot-parachain binary '~/local_bridge_testing/bin/polkadot-parachain' does not exist!"
-        echo "  You need to build it and copy to this location!"
-        echo "  Please, check ./parachains/runtimes/bridge-hubs/README.md (Prepare/Build/Deploy)"
-        exit 1
-    fi
+function relayer_path() {
+    local default_path=~/local_bridge_testing/bin/substrate-relay
+    local path="${SUBSTRATE_RELAY_PATH:-$default_path}"
+    echo "$path"
 }
 
 function ensure_relayer() {
-    if [[ ! -f ~/local_bridge_testing/bin/substrate-relay ]]; then
-        echo "  Required substrate-relay binary '~/local_bridge_testing/bin/substrate-relay' does not exist!"
+    local path=$(relayer_path)
+    if [[ ! -f "$path" ]]; then
+        echo "  Required substrate-relay binary '$path' does not exist!"
         echo "  You need to build it and copy to this location!"
         echo "  Please, check ./parachains/runtimes/bridge-hubs/README.md (Prepare/Build/Deploy)"
         exit 1
     fi
+
+    echo $path
 }
 
 function ensure_polkadot_js_api() {