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_class.vim b/src/testdir/test_vim9_class.vim
index 3a434d1..023068c 100644
--- a/src/testdir/test_vim9_class.vim
+++ b/src/testdir/test_vim9_class.vim
@@ -48,7 +48,7 @@
   lines =<< trim END
       vim9script
       class Something
-      endclass school's out 
+      endclass school's out
   END
   v9.CheckScriptFailure(lines, 'E488:')
 
@@ -623,7 +623,7 @@
       TextPos.AddToCounter(3)
       assert_equal(3, TextPos.counter)
       assert_fails('echo TextPos.noSuchMember', 'E1338:')
-      
+
       def GetCounter(): number
         return TextPos.counter
       enddef