Problem:    Tests failing because there is no error for float to string
            conversion.
Solution:   Change the check for failure to check for correct result.  Make
            some conversions strict in Vim9 script.

https://github.com/vim/vim/commit/3cfa5b16b06bcc034f6de77070fa779d698ab5e9

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Problem:    Slightly inconsistent error messages.
Solution:   Make it "Using a Float". (closes vim/vim#10959)

https://github.com/vim/vim/commit/dde77a7c4d72622284dc5fb72557bde42c314fa6

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Problem:    Inconsistent capitalization in error messages.
Solution:   Make capitalization consistent. (Doug Kearns)

https://github.com/vim/vim/commit/cf030578b26460643dca4a40e7f2e3bc19c749aa

Co-authored-by: Bram Moolenaar <Bram@vim.org>

Problem:    Vim9: builtin function arguments not checked at compile time.
Solution:   Add more type checks. (Yegappan Lakshmanan, closes vim/vim#8539)

https://github.com/vim/vim/commit/5b73992d8f82be7ac4b6f46c17f53ffb9640e5fa

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

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- `redir_exec` is obsolete, but it keeps getting used in new tests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