tests: Optimize 'fold-available-packages' test.

This test goes from 58s to 10s wall-clock time.

Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

* tests/packages.scm ("fold-available-packages with/without cache"):
Remove 'find-duplicates'.  Add 'list->set*' and use it instead of
'find-duplicates', 'delete-duplicates', and 'lset='.