Make displayhook in signal test kernel app

Recent changes in ipykernel have made the tests fail because the
IPKernelApp subclass used in tests was missing a displayhook attribute.

Author: takluyver

jupyter_client/tests/signalkernel.py

@@ -9,6 +9,7 @@
 import sys
 import time
 
+from ipykernel.displayhook import ZMQDisplayHook
 from ipykernel.kernelbase import Kernel
 from ipykernel.kernelapp import IPKernelApp
 
@@ -61,7 +62,8 @@ def kernel_info_request(self, *args, **kwargs):
 class SignalTestApp(IPKernelApp):
     kernel_class = SignalTestKernel
     def init_io(self):
-        pass # disable stdout/stderr capture
+        # Overridden to disable stdout/stderr capture
+        self.displayhook = ZMQDisplayHook(self.session, self.iopub_socket)
 
 if __name__ == '__main__':
     # make startup artificially slow,