Generated unit tests fail when method signatures contain multiple nested fields

[parthea]

From AIP-4232,

An RPC with the google.api.method_signature annotation indicates that an overload with a flattened method signature is desired where supported. The string contains comma-separated arguments, in order. If a field's name contains a period (.) character, this indicates a nested field

When there are multiple nested fields in the method signature, test*flattened unit tests fail with AssertionError. See the stack trace below from this build log in PR #1954 at commit b27c2d80f9b68c8baed1ccfe83474ad04d6759fc.

.......................F.......................................          [100%]
=================================== FAILURES ===================================
___________________________ test_my_method_flattened ___________________________

    def test_my_method_flattened():
        client = MyServiceClient(
            credentials=ga_credentials.AnonymousCredentials(),
        )
    
        # Mock the actual call within the gRPC stub, and fake the request.
        with mock.patch.object(
                type(client.transport.my_method),
                '__call__') as call:
            # Designate an appropriate return value for the call.
            call.return_value = test_flattened_value.MethodResponse()
            # Call the method with a truthy value for each flattened field,
            # using the keyword arguments to the method.
            client.my_method(
                parameter=struct_pb2.Value(null_value=struct_pb2.NullValue.NULL_VALUE),
                items=[struct_pb2.Value(null_value=struct_pb2.NullValue.NULL_VALUE)],
                some_message=test_flattened_value.MethodRequest.SomeNestedMessage(foo='foo_value'),
                foo='foo_value',
                bar='bar_value',
            )
    
            # Establish that the underlying call was made with the expected
            # request object values.
            assert len(call.mock_calls) == 1
            _, args, _ = call.mock_calls[0]
            arg = args[0].parameter
            mock_val = struct_pb2.Value(null_value=struct_pb2.NullValue.NULL_VALUE)
            from proto.marshal import Marshal
            from proto.marshal.rules.struct import ValueRule
            rule = ValueRule(marshal=Marshal(name="Test"))
            mock_val = rule.to_python(mock_val)
            assert arg == mock_val
            arg = args[0].items
            mock_val = [struct_pb2.Value(null_value=struct_pb2.NullValue.NULL_VALUE)]
            assert arg == mock_val
            arg = args[0].some_message
            mock_val = test_flattened_value.MethodRequest.SomeNestedMessage(foo='foo_value')
>           assert arg == mock_val
E           assert foo: "foo_value"\nbar: "bar_value"\n == foo: "foo_value"\n

/tmp/tmp3su_4fjt/tests/unit/gapic/fragment/test_my_service.py:536: AssertionError

See b/326213033 and cl/606967264 where the issue was discovered

[parthea]

Bumping to p3 as we don't currently have any APIs that are blocked on https://github.com/googleapis/gapic-generator-python/issues/1955