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").
3 jobs for modesetting-manpage in 5 minutes and 54 seconds
Status Job ID Name Coverage
  Docker Image
passed #693673
debian-testing

00:00:20

 
  Build And Test
passed #693674
autotools-build-and-test

00:05:34

passed #693675
meson-build-and-test

00:01:21