patch 8.1.1840: Testing: WorkingClipboard() is not accurate

Problem:    Testing: WorkingClipboard() is not accurate.
Solution:   Check feature clipboard_working instead.
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 475a09a..75eaa7b 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -182,9 +182,7 @@
 endfunc
 
 func Test_term_mouse_middle_click()
-  if !WorkingClipboard()
-    throw 'Skipped: No working clipboard'
-  endif
+  CheckFeature clipboard_working
 
   new
   let save_mouse = &mouse