Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

Ensure that
  * Shell uses UTF8 input/output mode
  * Stderr output is captured, in UTF8
  * Program exit codes are correctly captured

Update functional test harness and add tests
for :make command.

Closes #13713

* tv_to_argv: error when cmd is not executable
  Callers always assume that emsg was emitted:
  - https://github.com/neovim/neovim/blob/57fbf288/src/nvim/eval.c#L12509
  - https://github.com/neovim/neovim/blob/57fbf288/src/nvim/eval.c#L17923
  - https://github.com/neovim/neovim/blob/57fbf288/src/nvim/eval.c#L18202
* test/functional/provider: display reason from missing_provider
* provider#node#Detect: skip / handle non-existing node executable

It is perfectly fine and expected to detach from the screen just by
the UI disconnecting from nvim or exiting nvim. Just keep detach() in
screen_basic_spec, to get some coverage of the detach method itself.

This avoids hang on failure in many situations (though one could argue
that detach() should be "fast", or at least "as fast as resize",
which works in press-return already).

Never use detach() just to change the size of the screen, try_resize()
method exists for that specifically.

Removed 'echo' alias because it does not behave like POSIX echo.

* tests: move os_kill to functional helpers

* tests: fix system_spec when run with clipboard manager

Replaces "xclip" with a dedicated helper program.

Fixes: https://github.com/neovim/neovim/issues/4900#issuecomment-501866842

Problem:    When user tries to exit with CTRL-C message is confusing.
Solution:   Only mention ":qa!" when there is a changed buffer. (closes vim/vim#4163)
https://github.com/vim/vim/commit/a84a3dd6635fcd2e07f510cba6a999585dcc381a

vim-patch:8.1.1052: test for CTRL-C message sometimes fails

Problem:    test for CTRL-C message sometimes fails
Solution:   Make sure there are no changed buffers.
https://github.com/vim/vim/commit/553e5a5c568e7d175b65b0472cd6d9843b25f4c8

vim-patch:8.1.1053: warning for missing return statement

Problem:    Warning for missing return statement. (Dominique Pelle)
Solution:   Add return statement.
https://github.com/vim/vim/commit/d6c3f1fa2b5e1dd7dc87cf608d72b84ad696b58f

It would previously fail with `set shell=sh` (no slash).

For the test itself we can just use a non-existing (fake) shell, because
it is only about the verbose output.

Ref: https://github.com/neovim/neovim/issues/9330

ref https://github.com/neovim/neovim/issues/9001#issuecomment-421843790

Steps to reproduce:
    :set verbose=9
    :call system(['echo'])
    E730: using List as a String