policy: fix message for invalid manifest specifier

Add test for invalid manifest specifier and fix the error message
which is missing a space ("singletrailing" instead of
"single trailing").

PR-URL: https://github.com/nodejs/node/pull/40574


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Voltrex <mohammadkeyvanzade94@gmail.com>