Fix rare test failure in nbtree_half_dead_pages

Fix rare test failure in nbtree_half_dead_pages

If auto-analyze kicks in at just the right moment, it can hold a
snapshot and prevent the VACUUM command in the test from removing the
deleted tuples. The test needs the tuples to be removed, otherwise no
half-dead page is generated. To fix, introduce a helper procedure to
wait for the removable cutoff to advance, like the one used in the
syscache-update-pruned test for similar purposes.

Thanks to Alexander Lakhin for reproducing and analyzing the test
failure, and Tom Lane for the report.

Discussion: https://www.postgresql.org/message-id/307198.1767408023@sss.pgh.pa.us