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_cmd_lists.vim b/src/testdir/test_cmd_lists.vim
index 8a7d54f..77ae85e 100644
--- a/src/testdir/test_cmd_lists.vim
+++ b/src/testdir/test_cmd_lists.vim
@@ -50,7 +50,7 @@
           \ 'interface',
           \ 'public',
           \ 'static',
-          \ 'this', 
+          \ 'this',
           \ 'type',
           \ '++',
           \ '--',