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

Hope this will make people using feed_command less likely: this hides bugs.
Already found at least two:

1. msgpackparse() will show internal error: hash_add() in case of duplicate
   keys, though it will still work correctly. Currently silenced.
2. ttimeoutlen was spelled incorrectly, resulting in option not being set when
   expected. Test was still functioning somehow though. Currently fixed.

In Vim's main_loop function, the main loop is

        while (!cmdwin
    #ifdef FEAT_CMDWIN
                || cmdwin_result == 0
    #endif
                )
        {
    ...
    #ifdef FEAT_EVAL
            /*
             * May perform garbage collection when waiting for a character, but
             * only at the very toplevel.  Otherwise we may be using a List or
             * Dict internally somewhere.
             * "may_garbage_collect" is reset in vgetc() which is invoked through
             * do_exmode() and normal_cmd().
             */
            may_garbage_collect = (!cmdwin && !noexmode);
    #endif
            /*
             * If we're invoked as ex, do a round of ex commands.
             * Otherwise, get and execute a normal mode command.
             */
            if (exmode_active)
            {
                if (noexmode)   /* End of ":global/path/visual" commands */
                    return;
                do_exmode(exmode_active == EXMODE_VIM);
            }
            else
                normal_cmd(&oa, TRUE);
        }

cmdwin_result is set to 0 before calling main_loop to handle the cmdwin
window and gets changed when the user causes a command to execute
(either through pressing <CR> or <C-c>).  This means that when the
cmdwin isn't active OR the user is still editing their command,
main_loop runs and main_loop calls normal_cmd with toplevel true as long
as exmode isn't active.

When the normal mode state was extracted in dae006a9, the conditions for
toplevel and may_garbage_collect were combined.  Since toplevel was set
to always ignore cmdwin, the v:count(1) variables were no longer being
updated when a command was prefixed with a count in the cmdwin.

Closes #5404