Support variable substitution and tryserver- and waterfall-only cmd line args.

scripts/slave/recipes/chromium.expected/dynamic_isolated_script_test.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "chromium.linux",
       "--builder",
       "Linux Tests",
@@ skipping 140 matching lines @@
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "read test spec",
     "~followup_annotations": [
       "@@@STEP_TEXT@path: [SLAVE_BUILD]/src/testing/buildbot/chromium.linux.json@@@",
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"Linux Tests\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"isolated_scripts\": [@@@",
       "@@@STEP_LOG_LINE@json.output@      {@@@",
+      "@@@STEP_LOG_LINE@json.output@        \"args\": [@@@",
+      "@@@STEP_LOG_LINE@json.output@          \"--correct-common-arg\"@@@",
+      "@@@STEP_LOG_LINE@json.output@        ], @@@",
       "@@@STEP_LOG_LINE@json.output@        \"isolate_name\": \"telemetry_gpu_unittests\", @@@",
-      "@@@STEP_LOG_LINE@json.output@        \"name\": \"telemetry_gpu_unittests\"@@@",
+      "@@@STEP_LOG_LINE@json.output@        \"name\": \"telemetry_gpu_unittests\", @@@",
+      "@@@STEP_LOG_LINE@json.output@        \"non_precommit_args\": [@@@",
+      "@@@STEP_LOG_LINE@json.output@          \"--these-args-should-be-present\", @@@",
+      "@@@STEP_LOG_LINE@json.output@          \"--test-machine-name=\\\"${buildername}\\\"\", @@@",
+      "@@@STEP_LOG_LINE@json.output@          \"--build-revision=\\\"${got_revision}\\\"\"@@@",
+      "@@@STEP_LOG_LINE@json.output@        ], @@@",
+      "@@@STEP_LOG_LINE@json.output@        \"precommit_args\": [@@@",
+      "@@@STEP_LOG_LINE@json.output@          \"--SHOULD-NOT-BE-PRESENT-DURING-THE-RUN\"@@@",
+      "@@@STEP_LOG_LINE@json.output@        ]@@@",
       "@@@STEP_LOG_LINE@json.output@      }@@@",
       "@@@STEP_LOG_LINE@json.output@    ]@@@",
       "@@@STEP_LOG_LINE@json.output@  }@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "cmd": [
       "python",
@@ skipping 66 matching lines @@
     "cmd": [
       "python",
       "-u",
       "[SLAVE_BUILD]/src/tools/swarming_client/run_isolated.py",
       "--isolated",
       "eeeeeeeeeeeeeeeeeeeeeeeeeeeeee",
       "-I",
       "https://isolateserver.appspot.com",
       "--verbose",
       "--",
+      "--correct-common-arg",
+      "--these-args-should-be-present",
+      "--test-machine-name=\"Linux Tests\"",
+      "--build-revision=\"f27fede2220bcd326aee3e86ddfd4ebd0fe58cb9\"",
       "--isolated-script-test-output",
       "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "telemetry_gpu_unittests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"failures\": [], @@@",
       "@@@STEP_LOG_LINE@json.output@  \"valid\": true@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]