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/gen_opt_test.vim b/src/testdir/gen_opt_test.vim
index faff33a..271abed 100644
--- a/src/testdir/gen_opt_test.vim
+++ b/src/testdir/gen_opt_test.vim
@@ -73,7 +73,7 @@
       \ 'buftype': [['', 'help', 'nofile'], ['xxx', 'help,nofile']],
       \ 'casemap': [['', 'internal'], ['xxx']],
       \ 'cedit': [['', '\<Esc>'], ['xxx', 'f']],
-      \ 'clipboard': [['', 'unnamed', 'autoselect,unnamed', 'html', 'exclude:vimdisplay'], ['xxx', '\ze*']],
+      \ 'clipboard': [['', 'unnamed', 'autoselect,unnamed', 'html', 'exclude:vimdisplay'], ['xxx', '\ze*', 'exclude:\\%(']],
       \ 'colorcolumn': [['', '8', '+2'], ['xxx']],
       \ 'comments': [['', 'b:#'], ['xxx']],
       \ 'commentstring': [['', '/*%s*/'], ['xxx']],