Test case for editor and mode

[gaocegege]

ref #10 (comment)

• Testing the editor: is there any specific editor testing that needs to be done, or should that be left to Processing?

I am not sure which layer we should put the test in. But if Processing does it, it works for Processing.R, too. And I will file a new issue about it and try to do a research.

• Testing the mode itself: perhaps open a separate issue (unscheduled, no milestone) for this to come back to it in the future.

Yeah, it is helpful. I am not sure if the mode test and the editor test are the same thing, we could put them in one issue.