Document the process display buffer nuances and add unit tests

[vspinu]

Let's keep track of the process display issues in one place and try to device a comprehensive test

Relevant issues:
#987 partially fixed by 21cb413
#1067 fixed by 0ee7582
#1074 consequence of 0ee7582 and fixed in 983c54b
#1040

I don't understand all the nuances/requirements of the display-buffer machinery in ESS. Nor I customize display-buffer-alist. @jabranham, @lionel- please feel free to pick this up :)

[lionel-]

I'm not familiar with display-buffer either. I started using it recently but only for prog files with a simple config. I think the most important thing is to set up unit tests, which might be a bit tricky since the windows/frame infrastructure is disabled in batch mode IIRC.