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

Eliminate `expect_err` in favor of `pcall_err` + `eq` or `matches`.

- Return VimL errors instead of generic errors for:
  - nvim_call_function
  - nvim_call_dict_function
- Fix tests which were silently broken before this change.

This violates #6150 where we agreed not to translate API errors.  But
that can be fixed later.

Before this change, if $NVIM_LISTEN_ADDRESS was invalid, v:servername
was left empty.

Travis disabled IPv6:

[ RUN      ] serverstart(), serverstop() parses endpoints correctly: FAIL
...build/neovim/neovim/test/functional/eval/server_spec.lua:83: Expected objects to be the same.
Passed in:
(table) {
  [1] = '127.0.0.1:12345' }
Expected:
(table) {
  [1] = '127.0.0.1:12345'
 *[2] = '::1:12345' }

Change all tests to ensure a server was actually started before
expecting it to be returned from serverlist().

In the process of setting up the socket watcher, the address may be
changed (e.g., adding the OS-selected port).