Add test for single-page VACUUM of hash index on INSERT

Add test for single-page VACUUM of hash index on INSERT

_hash_vacuum_one_page() in hashinsert.c is a routine related to hash
indexes that can perform a single-page VACUUM when dead tuples are
detected during index insertion.  This routine previously had no test
coverage, and this commit adds a test case for that purpose.

To safely create dead tuples in a way that works with parallel tests,
this uses a technique based on a rollbacked INSERT, following a
suggestion by Heikki Linnakangas.

Author: Alexander Kuzmenkov <akuzmenkov@tigerdata.com>
Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CALzhyqxrc1ZHYmf5V8NE+yMboqVg7xZrQM7K2c7VS0p1v8z42w@mail.gmail.com