gitlab-ci: Add a manpage substitutions regression test
This catches the broken manpages in the autoconf build which appeared
after commit 2e497bf8 ("man: s/__/@/g") and were only partly
rectified by commit 0445705a ("man: Fix automake seddery").
Reviewed-by: Michel Dänzer <firstname.lastname@example.org>
6 minutes and 46 seconds
(queued for 26 seconds)