Willy made me realize that tree-based matching may also suffer from
out-of-order mapfile loading, as opposed to what's being said in
b546bb6d ("BUG/MINOR: map: list-based matching potential ordering
regression") and the associated REGTEST.
Indeed, in case of duplicated keys, we want to be sure that only the key
that was first seen in the file will be returned (as long as it is not
removed). The above fix is still valid, and the list-based match regtest
will also prevent regressions for tree-based match since mapfile loading
logic is currently match-type agnostic.
But let's clarify that by making both the code comment and the regtest
more precise.
As shown in "BUG/MINOR: map: list-based matching potential ordering
regression", list-based matching types such as dom are affected by the
order in which elements are loaded from the map.
Since this is historical behavior and existing usages depend on it, we
add a test to prevent future regressions.
