[RFC] default to 'nocompatible'

I'm not really that happy with the changes I had to make to the first test in order to get it to work, so maybe someone else has some ideas to make it a bit better.

For the "this shouldn't be deleted" test, the actual output doesn't really make much sense (the line that says it should be deleted is not deleted, and the one that says it should not be modified is modified), so I made it expect the output that makes sense
Right..

I think this block will also be called if `:set nocp` is executed, so we can't use an assert. Can we do something like this (untested)?

```c
// Return error when trying to set 'compatible'.
if ((int *)varp == &p_cp && p_cp == TRUE) {
  p_cp = FALSE;
  return e_compatible_not_supported;
}
```

`:set nocp` would do nothing.