Disable implicit JS test asset generation

The JS test assets depend in a brittle way on the details of the tflow()
utility functions. We shouldn't have to fix JS tests when adjusting these.

Options:
    - Manually generate the test assets in a script.
    - Define the JS assets without using tflow, so they don't unexpextedly
    vary.

test/mitmproxy/tools/web/test_app.py

@@ -322,7 +322,7 @@ class TestApp(tornado.testing.AsyncHTTPTestCase):
         ws_client2 = yield websocket.websocket_connect(ws_url)
         ws_client2.close()
 
-    def test_generate_tflow_js(self):
+    def _test_generate_tflow_js(self):
         _tflow = app.flow_to_json(tflow.tflow(resp=True, err=True))
         # Set some value as constant, so that _tflow.js would not change every time.
         _tflow['client_conn']['id'] = "4a18d1a0-50a1-48dd-9aa6-d45d74282939"

web/src/js/__tests__/ducks/_tflow.js

@@ -45,8 +45,8 @@ export default function(){
         "port": 22,
         "pretty_host": "address",
         "scheme": "http",
-        "timestamp_end": 2,
-        "timestamp_start": 1
+        "timestamp_end": null,
+        "timestamp_start": null
     },
     "response": {
         "contentHash": "ab530a13e45914982b79f9b7e3fba994cfd1f3fb22f71cea1afbf02b460c6d1d",