lldp: add test coverage for sd_lldp_get_neighbors() with multiple neighbors

lldp: add test coverage for sd_lldp_get_neighbors() with multiple neighbors

In particular, check that the order of the results is consistent.

This test coverage will be useful in order to refactor the compare_func
used while sorting the results.

When introduced, this test also uncovered a memory leak in sd_lldp_stop(),
which was then fixed by a separate commit using a specialized function
as destructor of the LLDP Hashmap.

Tested:
  $ ninja -C build/ test
  $ valgrind --leak-check=full build/test-lldp