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_listchars.vim b/src/testdir/test_listchars.vim
index b9b078c..2f495c3 100644
--- a/src/testdir/test_listchars.vim
+++ b/src/testdir/test_listchars.vim
@@ -506,7 +506,7 @@
   set list
 
   set listchars=eol:$,space:_,nbsp:=
-  
+
   let nbsp1 = nr2char(0xa0)
   let nbsp2 = nr2char(0x202f)
   call append(0, [