[driver] output verbose logging from flutter driver

scripts/slave/recipes/flutter/flutter.expected/mac.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\nfrom common import chromium_utils # Error? See https://crbug.com/584783.\n\npath = sys.argv[1]\nif os.path.exists(path):\n  for p in (os.path.join(path, x) for x in os.listdir(path)):\n    if os.path.isdir(p):\n      chromium_utils.RemoveDirectory(p)\n    else:\n      os.unlink(p)\n",
       "[SLAVE_BUILD]"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "rmcontents everything",
@@ skipping 285 matching lines @@
     "env": {
       "ANDROID_HOME": "[SLAVE_BUILD]/flutter/infra/android_tools",
       "PATH": "[SLAVE_BUILD]/flutter/bin:[SLAVE_BUILD]/flutter/bin/cache/dart-sdk/bin:%(PATH)s",
       "PUB_CACHE": "[SLAVE_BUILD]/.pub-cache"
     },
     "name": "test create"
   },
   {
     "cmd": [
       "flutter",
-      "drive"
+      "drive",
+      "--verbose"
     ],
     "cwd": "[TMP_BASE]/tmp_tmp_1/sample_app",
     "env": {
       "ANDROID_HOME": "[SLAVE_BUILD]/flutter/infra/android_tools",
       "PATH": "[SLAVE_BUILD]/flutter/bin:[SLAVE_BUILD]/flutter/bin/cache/dart-sdk/bin:%(PATH)s",
       "PUB_CACHE": "[SLAVE_BUILD]/.pub-cache"
     },
     "name": "drive sample_app"
   },
   {
@@ skipping 20 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@  chromium_utils.RemoveDirectory(sys.argv[1])@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]