build: enable lintlua for test/unit/ dir #26396

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 lintlua for `test/unit/` directory.
- TODO: only `test/functional/` remains unchecked.

previous: 45fe4d11ad
previous: 517f0cc634

test/unit/eval/tricks_spec.lua

@@ -8,8 +8,7 @@ local eq = helpers.eq
 
 local eval0 = eval_helpers.eval0
 
-local eval = cimport('./src/nvim/eval.h', './src/nvim/eval/typval.h',
-                     './src/nvim/memory.h')
+local eval = cimport('./src/nvim/eval.h', './src/nvim/eval/typval.h', './src/nvim/memory.h')
 
 describe('NULL typval_T', function()
   itp('is produced by $XXX_UNEXISTENT_VAR_XXX', function()