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_functions.vim b/src/testdir/test_functions.vim
index 2492c93..5e4eacd 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -973,6 +973,7 @@
   call assert_equal(4,  match('testing', 'ing', -1))
   call assert_fails("let x=match('testing', 'ing', 0, [])", 'E745:')
   call assert_equal(-1, match(test_null_list(), 2))
+  call assert_equal(-1, match('abc', '\\%('))
 endfunc
 
 func Test_matchend()