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 <mdaenzer@redhat.com>
3 jobs for modesetting-manpage in 6 minutes and 46 seconds (queued for 26 seconds)
Status Job ID Name Coverage
  Docker Image
passed #696048
debian-testing

00:00:37

 
  Build And Test
passed #696049
autotools-build-and-test

00:06:09

passed #696050
meson-build-and-test

00:01:12