The vim_input function accepts raw terminal input and so is better to emulate
real user, especially because it is not deferred as vim_feedkeys.
Using this function required a number of changes:
- expect() was refactored to use curbuf_contents()
- The vim_eval function in request() was moved to curbuf_contents(). For most
cases this is enough(we only care for synchronizing api calls with user input
when verifying buffer contents).
- <C-@>(NUL) is preprocessed before being passed to replace_termcodes.
- Legacy test 4 had a bug that only became visible when using vim_input, it is
fixed now.
- An extra blank line deletion was required for test 101
The last two items show that vim_feedkeys because it is not 100% equivalent to
receiving terminal input.
