test: Refactor functional helpers to use vim_input

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.

test/functional/shell/viml_system_spec.lua

@@ -10,6 +10,7 @@ local eq, clear, eval, feed, nvim =
 local function create_file_with_nuls(name)
   return function()
     feed('ipart1<C-V>000part2<C-V>000part3<ESC>:w '..name..'<CR>')
+    eval('1')  -- wait for the file to be created
   end
 end