Add toast-level reloption for vacuum_index_cleanup

Add toast-level reloption for vacuum_index_cleanup

a96c41f has introduced the option for heap, but it still lacked the
variant to control the behavior for toast relations.

While on it, refactor the tests so as they stress more scenarios with
the various values that vacuum_index_cleanup can use.  It would be
useful to couple those tests with pageinspect to check that pages are
actually cleaned up, but this is left for later.

Author: Masahiko Sawada, Michael Paquier
Reviewed-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAD21AoCqs8iN04RX=i1KtLSaX5RrTEM04b7NHYps4+rqtpWNEg@mail.gmail.com