Expect INTERNAL in Python negative interop client

The test now expects to receive StatusCode.INTERNAL when it receives a
RST_STREAM from the server in order to comply with
https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md

Also added wait for ready behavior on the client channel.

src/python/grpcio_tests/tests/http2/negative_http2_client.py

@@ -82,20 +82,22 @@ def _goaway(stub):
 
 def _rst_after_header(stub):
     resp_future = stub.UnaryCall.future(_SIMPLE_REQUEST)
-    _validate_status_code_and_details(resp_future, grpc.StatusCode.UNAVAILABLE,
-                                      "")
+    _validate_status_code_and_details(resp_future, grpc.StatusCode.INTERNAL,
+                                      "Received RST_STREAM with error code 0")
 
 
 def _rst_during_data(stub):
     resp_future = stub.UnaryCall.future(_SIMPLE_REQUEST)
-    _validate_status_code_and_details(resp_future, grpc.StatusCode.UNKNOWN, "")
+    _validate_status_code_and_details(resp_future, grpc.StatusCode.INTERNAL,
+                                      "Received RST_STREAM with error code 0")
 
 
 def _rst_after_data(stub):
     resp_future = stub.UnaryCall.future(_SIMPLE_REQUEST)
     _validate_payload_type_and_length(
         next(resp_future), messages_pb2.COMPRESSABLE, _RESPONSE_SIZE)
-    _validate_status_code_and_details(resp_future, grpc.StatusCode.UNKNOWN, "")
+    _validate_status_code_and_details(resp_future, grpc.StatusCode.INTERNAL,
+                                      "Received RST_STREAM with error code 0")
 
 
 def _ping(stub):
@@ -159,6 +161,7 @@ def _args():
 def _stub(server_host, server_port):
     target = '{}:{}'.format(server_host, server_port)
     channel = grpc.insecure_channel(target)
+    grpc.channel_ready_future(channel).result()
     return test_pb2.TestServiceStub(channel)