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.
diff --git a/src/testdir/test_vim9_import.vim b/src/testdir/test_vim9_import.vim
index 97d1cff..7d3691a 100644
--- a/src/testdir/test_vim9_import.vim
+++ b/src/testdir/test_vim9_import.vim
@@ -2656,7 +2656,7 @@
 
       import autoload 'toggle.vim'
 
-      nnoremap <silent> <expr> tt toggle.Toggle() 
+      nnoremap <silent> <expr> tt toggle.Toggle()
       nnoremap <silent> xx <ScriptCmd>toggle.Doit()<CR>
       nnoremap <silent> yy <Cmd>toggle.Doit()<CR>
   END