fairland: Make planned island sizes fair

fairland: Make planned island sizes fair

The previous two commits put the same number of islands closest to
each continent.  This one makes their planned sizes match: instead of
rolling dice separately for each island's size, we roll dice only for
the first continent's islands.  The other continent's islands get the
same sizes.

Actual island sizes still differ when islands can't be grown to their
planned size.  To be addressed next.

fairland-test needs a tweak to avoid loss of test coverage.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>