t/re/reg_mesg.t: Add override of warning default on/off

t/re/reg_mesg.t: Add override of warning default on/off

This .t needs an overhaul to more cleanly accommodate the extra tasks it
has been given over the years.  But until then, this is a minimal
enhancement that will be useful in the commit after this one.

This adds the ability to specify that a particular pattern being tested
should generate a message which is raised by default vs one that isn't.
The messages are currently grouped in categories whose default is
determined by the category itself.  This commit avoids having to create
a new category when a message comes along that doesn't quite fit into
the existing ones.