build: enable lintlua for test/ dir

Problem:
Not all Lua code is checked by stylua. Automating code-style is an
important mechanism for reducing time spent on accidental
(non-essential) complexity.

Solution:
- Enable stylua for entire `test/` directory.
- Exclude these high-churn files until this issue is resolved: https://github.com/JohnnyMorganz/StyLua/issues/829
  ```
  test/functional/ui/decorations_spec.lua  | 3560 ++++++++++++++++++++++++++++++++++++----------------
  test/functional/ui/float_spec.lua        | 5826 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-------------------------
  test/functional/ui/multigrid_spec.lua    | 1349 ++++++++++++++------
  ```
- Make surgical changes to these files (or add `stylua: ignore` in some
  small scopes) to improve the result:
  ```
  test/functional/vimscript/msgpack_functions_spec.lua | 1414 +++++++++++++++------
  test/functional/api/buffer_spec.lua                  | 1389 +++++++++++----------
  test/functional/api/vim_spec.lua                     | 2740 +++++++++++++++++++++++-----------------
  ```
- These "high churn" files are NOT excluded because the changes are
  largely an improvement:
  ```
  test/functional/plugin/lsp_spec.lua      | 2198 ++++++++++++++++++---------------
  test/functional/plugin/shada_spec.lua    | 4078 +++++++++++++++++++++++++++++++++++-------------------------
  test/functional/ui/cmdline_spec.lua      | 1199 +++++++++++-------
  test/functional/ui/popupmenu_spec.lua    | 1267 +++++++++++--------
  test/functional/ui/messages_spec.lua     | 1643 +++++++++++++++---------
  ```
- TODO: how to check "all directories"? With `GLOB_DIRS *` and `/.deps/` (or
  `.deps/`) in `.styluaignore`, Lua code in `.deps/` is still checked...

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