run_test.py: Support to inject Python code, useful for deployment

Needed for: elastic/elasticsearch#30112 (comment)

> I already have a workaround in place for this which consists of automatically deploying as many watches as I need to send different mails. Those watches are derived from my watch definition. For this, I extended [run_test.py](https://github.com/elastic/examples/blob/master/Alerting/Sample%20Watches/run_test.py) to inject Python code after the watch definition is read. Not ideal, but it is maintainable.

Alerting/Sample Watches/run_test.py

@@ -33,6 +33,7 @@ def load_file(serialized_file):
     parser.add_argument('--protocol', help='protocol')
     parser.add_argument('--test_file', help='test file')
     parser.add_argument('--keep-index', help='Keep the index where test documents have been loaded to after the test', action='store_true')
+    parser.add_argument('--modify-watch-by-eval', help='Python code to modify the watch before loading it into Elastic')
     parser.add_argument(
         '--no-execute-watch',
         help='Do not force watch execution. This can be useful when you use this script to deploy the watch.',
@@ -79,6 +80,10 @@ def load_file(serialized_file):
 
     # Load Watch and Execute
     watch = load_file(test['watch_file'])
+
+    if args.modify_watch_by_eval:
+        eval(compile(args.modify_watch_by_eval, '<string>', 'exec'))
+
     watcher = XPackClient(es).watcher
     watcher.put_watch(id=test["watch_name"], body=watch)