give --task-args option in test_runner a help description (#133791)

Fixes flutter/flutter#133790

Provides help text for the `--task-args` option of the `test_runner` devicelab command. The current help text is just copypasta from another option's help text

dev/devicelab/lib/command/test.dart

@@ -13,7 +13,7 @@ class TestCommand extends Command<void> {
         help: 'The name of a task listed under bin/tasks.\n'
             '   Example: complex_layout__start_up.\n');
     argParser.addMultiOption('task-args',
-        help: 'The name of a task listed under bin/tasks.\n'
+        help: 'List of arguments to pass to the task.\n'
             'For example, "--task-args build" is passed as "bin/task/task.dart --build"');
     argParser.addOption(
       'device-id',