Followup to #21055: regression tests for the code that
reads man pages. These are not xref-related at all, just
simple consistency checks on the man page content.
In the process of writing these tests, I also fixed a
longstanding bug where warning messages could be emitted
multiple times, once for each time we read a man page file
(as happens with command aliases).
Signed-off-by: Ed Santiago <santiago@redhat.com>
