Perl: fix not setting TRACK_FLAG when encoding with aliased_dedupe_strings

See https://github.com/Sereal/Sereal/issues/46

* always set the track_bit to the alias position when aliased_deduping
* add simple test case for aliased_dedupe that checks that the items
  acutaly have the same refaddrs
* fix the TestSet's @BasicTests to support adding track_flag at
  short_string(), so the aliased_dedupe tests from 010_desperate.t
  works correctly.