Add support for swarming priority and expiration in the test spec

scripts/slave/recipe_modules/swarming/example.expected/basic_0.4.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::git]/resources/git_setup.py",
       "--path",
       "[SLAVE_BUILD]/swarming.client",
       "--url",
       "https://chromium.googlesource.com/external/swarming.client.git"
@@ skipping 147 matching lines @@
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/swarming.client/swarming.py",
       "trigger",
       "--swarming",
       "https://chromium-swarm-dev.appspot.com",
       "--isolate-server",
       "https://isolateserver-dev.appspot.com",
       "--priority",
-      "30",
+      "125",
       "--shards",
       "1",
       "--task-name",
       "hello_world/Windows-7-SP1/hash_for_w",
       "--dump-json",
       "/path/to/tmp/json",
       "--expiration",
       "3600",
       "--io-timeout",
       "1200",
@@ skipping 50 matching lines @@
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/swarming.client/swarming.py",
       "trigger",
       "--swarming",
       "https://chromium-swarm-dev.appspot.com",
       "--isolate-server",
       "https://isolateserver-dev.appspot.com",
       "--priority",
-      "30",
+      "255",
       "--shards",
       "2",
       "--task-name",
       "hello_world/Ubuntu-12.04/hash_for_l",
       "--dump-json",
       "/path/to/tmp/json",
       "--expiration",
       "3600",
       "--io-timeout",
       "1200",
@@ skipping 376 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
   }
 ]