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/benchmark/bench_regexp_spec.lua

@@ -10,8 +10,7 @@ local result_file = 'benchmark.out'
 local sample_file = 'test/old/testdir/samples/re.freeze.txt'
 
 -- Vim script code that does both the work and the benchmarking of that work.
-local measure_cmd =
-    [[call Measure(%d, ']] .. sample_file .. [[', '\s\+\%%#\@<!$', '+5')]]
+local measure_cmd = [[call Measure(%d, ']] .. sample_file .. [[', '\s\+\%%#\@<!$', '+5')]]
 local measure_script = [[
     func Measure(re, file, pattern, arg)
       let sstart = reltime()