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_statusline.vim b/src/testdir/test_statusline.vim
index d06e903..8061d15 100644
--- a/src/testdir/test_statusline.vim
+++ b/src/testdir/test_statusline.vim
@@ -413,7 +413,7 @@
   " Test statusline works with 80+ items
   function! StatusLabel()
     redrawstatus
-    return '[label]'	
+    return '[label]'
   endfunc
   let statusline = '%{StatusLabel()}'
   for i in range(150)