Add tests specifically for axmap_next_free()

Add tests for axmap_next_free() to ensure the new search algorithm
works correctly.  Since the new algorithm always looks forward for
the next free bit, it's possible to test for specific results given
a known condition of the map.  Test maps that are mostly empty and
mostly full. Test wrap-around. Test maps of particular sizes that
have unused bits in middle levels as well as in level 0.