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_virtualedit.vim

@@ -696,14 +696,14 @@ func Test_virtualedit_mouse()
   set virtualedit&
 endfunc
 
-" this was replacing the NUL at the end of the line 
+" this was replacing the NUL at the end of the line
 func Test_virtualedit_replace_after_tab()
   new
   s/\v/	0
   set ve=all
   let @" = ''
   sil! norm vPvr0
-  
+
   call assert_equal("\t0", getline(1))
   set ve&
   bwipe!