Bridge zombienet tests: move all "framework" files under one folder (#3462)

Related to https://github.com/paritytech/polkadot-sdk/issues/3400

Moving all bridges testing "framework" files under one folder in order
to be able to download the entire folder when we want to add tests in
other repos

No significant functional changes

bridges/testing/framework/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
+}