Handle failure to read JSON waiting for server start

If the test controller tries to read the file while the server it has
started is in the middle of writing it, it gets invalid JSON and fails.

This just loops again until we have valid JSON to read.

IPython/testing/iptestcontroller.py

@@ -274,8 +274,14 @@ class JSController(TestController):
             if self.server.poll() is not None:
                 return self._failed_to_start()
             if os.path.exists(self.server_info_file):
-                self._load_server_info()
-                return
+                try:
+                    self._load_server_info()
+                except ValueError:
+                    # If the server is halfway through writing the file, we may
+                    # get invalid JSON; it should be ready next iteration.
+                    pass
+                else:
+                    return
             time.sleep(0.1)
         print("Notebook server-info file never arrived: %s" % self.server_info_file,
             file=sys.stderr