input: Refactor to ensure user input has higher priority
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: Fix problems in job_spec.lua

Nvim wasn't exiting cleanly in some job tests due to errors.

This can't be noticed until the next commit, which will perform a refactoring to
selectively process K_EVENT, so the `qa!` command executed at the end of each
test blocks forever if there are errors which require the user to press ENTER(in
that state Nvim no longer will process events).
event: No longer process K_EVENT automatically

Two new functions, `event_enable_deferred()`/`event_disable_deferred()` have to
be called by code that is capable of handling asynchronicity. User-dialog states
like "press ENTER to continue" or the swap file confirmation no longer will
generate K_EVENT.
Merge PR #1518 'Improve functional tests and perform explicit K_EVENT handling'