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_method.vim

@@ -128,7 +128,7 @@ endfunc
 
 func Test_method_syntax()
   eval [1, 2, 3]  ->sort( )
-  eval [1, 2, 3]  
+  eval [1, 2, 3]
 	\ ->sort(
 	\ )
   call assert_fails('eval [1, 2, 3]-> sort()', 'E15:')