Add tests for GC_register/move/unregister_long_link

Add tests for GC_register/move/unregister_long_link

* tests/test.c (live_long_refs): New global variable (only if
not GC_LONG_REFS_NOT_NEEDED and not MACOS).
* tests/test.c (mktree): Test GC_REGISTER_LONG_LINK,
GC_move_long_link, GC_unregister_long_link (using live_long_refs)
unless GC_LONG_REFS_NOT_NEEDED.
* tests/test.c (check_heap_stats): Define "still_long_live" local
variable, count non-null elements of live_long_refs, print number of
non-null elements of live_long_refs (if any) unless
GC_LONG_REFS_NOT_NEEDED.