Fixed #31224 -- Added support for asynchronous views and middleware.

This implements support for asynchronous views, asynchronous tests,
asynchronous middleware, and an asynchronous test client.

tests/test_client/views.py

@@ -25,6 +25,10 @@ def get_view(request):
     return HttpResponse(t.render(c))
 
 
+async def async_get_view(request):
+    return HttpResponse(b'GET content.')
+
+
 def trace_view(request):
     """
     A simple view that expects a TRACE request and echoes its status line.