Match Vim's behavior.

Problem:    Autocommand test fails on MS-Windows.
Solution:   Use pathcmp() instead of strcmp() to check if a directory differs.
https://github.com/vim/vim/commit/9eb76af451ddd8eaad0cd5dd629f18c4f4035171

Problem:    DirChanged is also triggered when the directory didn't change.
            (Daniel Hahler)
Solution:   Compare the current with the new directory. (closes vim/vim#3697)
https://github.com/vim/vim/commit/2caad3fbbdbf1486a176c9f6bfbc3d9be90e09f7

Problem
- `redir_exec` is obsolete, but it keeps getting used in new tests
  because people copy existing tests.
- Disadvantages of `redir_exec`:
  - Captures extra junk before the actual error/message that we _want_ to test.
  - Does not fail on error, unlike e.g. `command()`.

Solution
- Use new functions like `nvim_exec` and `pcall_err`.

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

Closes #4713

It is wrong to assume that you can't shorten a path if it's in `/`: you
can always shorten it by removing the forward slash.

Closes #14512

Implement BufModifiedSet autocmd