Add a CRUD test (create/fetch/update/delete) plus fetchAllFor() for
the Comment class, which had no test coverage yet.
Comment is generic/polymorphic (fk_element + element_type, no DB
foreign key), used by core/actions_comments.inc.php to power the
"Comments" tab on several object cards. The test attaches a comment
to a freshly created Project, matching that real usage.
No module activation needed here: llx_comment is a core table, not
gated behind any activatable module.
