Add very basic test for kill_prior_tuples

Add very basic test for kill_prior_tuples

Previously our tests did not exercise kill_prior_tuples for hash and gist. For
gist some related paths were reached, but gist's implementation seems to not
work if all the dead tuples are on one page (or something like that). The
coverage for other index types was rather incidental.

Thus add an explicit test ensuring kill_prior_tuples works at all.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/lxzj26ga6ippdeunz6kuncectr5gfuugmm2ry22qu6hcx6oid6@lzx3sjsqhmt6