patch 8.2.1593: tests do not check the error number properly

Problem:    Tests do not check the error number properly.0
Solution:   Add a colon after the error number. (closes #6869)
diff --git a/src/testdir/test_winbar.vim b/src/testdir/test_winbar.vim
index 86066af..3412ce5 100644
--- a/src/testdir/test_winbar.vim
+++ b/src/testdir/test_winbar.vim
@@ -18,7 +18,7 @@
   call assert_equal(12, g:did_cont)
 
   wincmd w
-  call assert_fails('emenu WinBar.Next', 'E334')
+  call assert_fails('emenu WinBar.Next', 'E334:')
   wincmd p
 
   aunmenu WinBar.Next