vim-patch:9.0.1257: code style is not check in test scripts

Problem:    Code style is not check in test scripts.
Solution:   Add basic code style check for test files.

94722c5107

Use Test_test_files() from latest Vim.

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

test/old/testdir/test_codestyle.vim

@@ -6,6 +6,45 @@ func s:ReportError(fname, lnum, msg)
   endif
 endfunc
 
+func Test_test_files()
+  for fname in glob('*.vim', 0, 1)
+    let g:ignoreSwapExists = 'e'
+    exe 'edit ' .. fname
+
+    " some files intentionally have misplaced white space
+    if fname =~ 'test_cindent.vim' || fname =~ 'test_join.vim'
+      continue
+    endif
+
+    " skip files that are known to have a space before a tab
+    if fname !~ 'test_comments.vim'
+          \ && fname !~ 'test_listchars.vim'
+          \ && fname !~ 'test_visual.vim'
+      call cursor(1, 1)
+      let skip = 'getline(".") =~ "codestyle: ignore"'
+      let lnum = search(fname =~ "test_regexp_latin" ? '[^á] \t' : ' \t', 'W', 0, 0, skip)
+      call s:ReportError('testdir/' .. fname, lnum, 'space before Tab')
+    endif
+
+    " skip files that are known to have trailing white space
+    if fname !~ 'test_cmdline.vim'
+          \ && fname !~ 'test_let.vim'
+          \ && fname !~ 'test_tagjump.vim'
+          \ && fname !~ 'test_vim9_cmd.vim'
+      call cursor(1, 1)
+      let lnum = search(
+            \ fname =~ 'test_vim9_assign.vim' ? '[^=]\s$'
+            \ : fname =~ 'test_vim9_class.vim' ? '[^)]\s$'
+            \ : fname =~ 'test_vim9_script.vim' ? '[^,:3]\s$'
+            \ : fname =~ 'test_visual.vim' ? '[^/]\s$'
+            \ : '[^\\]\s$')
+      call s:ReportError('testdir/' .. fname, lnum, 'trailing white space')
+    endif
+  endfor
+
+  bwipe!
+endfunc
+
 func Test_help_files()
   set nowrapscan