c2513365 has added some tests to check if a toast relation should be
empty or not, hardcoding the toast relation name when calling
pg_relation_size().  pg_class.reltoastrelid offers the same information,
so simplify the tests to use that.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20190403065949.GH3298@paquier.xyz