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

Given the other type names "jumps", "vars", etc., the name "buflist"
is somewhat unintuitive.

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

- Always load files when cleaning up jumplist.
  - For Shada: avoids writing duplicate entries, which happens when you read
    from a shada file with duplicate entries (merging the jumplist while
    writing sometimes produces duplicate entries, bug?) and then write right
    away (i.e.: without any `:jumps`, `getjumplist()`, or any jump movement,
    that is: nothing that calls `cleanup_jumplist` with `loadfiles == true`).
  - For Context: avoids non-idempotent behavior for the same reason (i.e.:
    first call to `shada_encode_jumps` does not remove duplicate entries).

- Do not set pcmark when dumping jumplist for Context.
  - Retrieving current Context shouldn't add an entry to the jumplist
    (which will be removed by a subsequent `cleanup_jumplist` anyway, i.e.:
    tail entry matching current position), just act like `getjumplist` for
    instance.