patch 8.2.1736: failure to compile a pattern not tested much

Problem:    Failure to compile a pattern not tested much.
Solution:   Add tests where a pattern fails to compile. (Yegappan Lakshmanan,
            closes #7004)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index 9b23a51..615d816 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -425,6 +425,7 @@
   call assert_equal(['b'], argv())
   call assert_fails('argdelete', 'E610:')
   call assert_fails('1,100argdelete', 'E16:')
+  call assert_fails('argdel /\)/', 'E55:')
 
   call Reset_arglist()
   args a b c d