Test for region printing

[MarcellPerger1]

We need tests for our fragile region-printing code.

And we need to decide how we want multi-line stuff to be displayed. I doubt there is much point underlining all off the internal lines...

Currently it is:

1 |  let whatever = (12 + a * 4
  |                 ^^^^^^^^^^^  
2 |      + 44 * 9 / 6 % 1
  |  ^^^^^^^^^^^^^^^^^^^^  
3 |      - hh + gg
  |  ^^^^^^^^^^^^^  
4 |      + 1.2 ** f);
  |  ^^^^^^^^^^^^^^^  

where I would prefer it to be

1 |  let whatever = (12 + a * 4
  |                 ^^^^^^^^^^^  
2 |      + 44 * 9 / 6 % 1
3 |      - hh + gg
4 |      + 1.2 ** f);
  |      ^^^^^^^^^^^ 

Or maybe

1 |  let whatever = (12 + a * 4
  |                 ^~~~~~~~~~~  
2 |      + 44 * 9 / 6 % 1
3 |      - hh + gg
4 |      + 1.2 ** f);
  |      ~~~~~~~~~~^ 

Originally posted by @MarcellPerger1 in #72 (comment)