Unit test request replay thread.

This is a small patch, but is the culmination of lots of work: we can now unit
test the deep innards of mitmproxy, with coverage. There's a lot more to come
in this vein.

libmproxy/flow.py

@@ -1382,6 +1382,7 @@ class FlowMaster(controller.Master):
                     self.masterq,
                 )
             rt.start() # pragma: no cover
+        return rt
 
     def run_script_hook(self, name, *args, **kwargs):
         if self.script and not self.pause_scripts:

test/test_server.py

@@ -20,6 +20,16 @@ class SanityMixin:
     def test_large(self):
         assert len(self.pathod("200:b@50k").content) == 1024*50
 
+    def test_replay(self):
+        assert self.pathod("304").status_code == 304
+        assert len(self.master.state.view) == 1
+        l = self.master.state.view[0]
+        assert l.response.code == 304
+        l.request.path = "/p/305"
+        rt = self.master.replay_request(l)
+        rt.join()
+        assert l.response.code == 305
+
 
 class TestHTTP(tutils.HTTPProxTest, SanityMixin):
     def test_invalid_http(self):