Adding this is issue 2 of #4177, changing the behavior when [find_path]
is given a SLF which matches multiple hexes.
The map and tests here should be easy enough for manually editing them. It
duplicates some of the functionality of the existing characterize_pathfinding
tests, however those tests need their expected values to be calculated and
can't be changed by hand.
'''nearest_by''': {DevFeature1.15|2} possible values "movement_cost"
(default), "steps", "hexes". If the [destination] SLF matches multiple hexes,
the one that would need the least movement points to reach may not be the one
that's closest as measured by '''hexes''', or closest as measured by steps,
from the starting point.
Behavior in 1.14 depended on which hex was checked first.
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