Bridge zombienet tests - move all test scripts to the same folder (#3333)

Related to https://github.com/paritytech/polkadot-sdk/issues/3242

bridges/testing/utils/common.sh

@@ -0,0 +1,45 @@
+#!/bin/bash
+
+function start_background_process() {
+    local command=$1
+    local log_file=$2
+    local __pid=$3
+
+    $command > $log_file 2>&1 &
+    eval $__pid="'$!'"
+}
+
+function wait_for_process_file() {
+    local pid=$1
+    local file=$2
+    local timeout=$3
+    local __found=$4
+
+    local time=0
+    until [ -e $file ]; do
+      if ! kill -0 $pid; then
+        echo "Process finished unsuccessfully"
+        return
+      fi
+      if (( time++ >= timeout )); then
+        echo "Timeout waiting for file $file: $timeout seconds"
+        eval $__found=0
+        return
+      fi
+      sleep 1
+    done
+
+    echo "File $file found after $time seconds"
+    eval $__found=1
+}
+
+function ensure_process_file() {
+    local pid=$1
+    local file=$2
+    local timeout=$3
+
+    wait_for_process_file $pid $file $timeout file_found
+    if [ "$file_found" != "1" ]; then
+      exit 1
+    fi
+}