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_cmdline.vim b/src/testdir/test_cmdline.vim
index a547ac4..026625e 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -185,6 +185,8 @@
   unmap <Left>
   set cpo-=k
 
+  call assert_fails('call feedkeys(":map \\\\%(\<Tab>\<Home>\"\<CR>", "xt")', 'E53:')
+
   unmap <Middle>x
   set cpo&vim
 endfunc