Add a test for incomplete splits in B-tree indexes

Add a test for incomplete splits in B-tree indexes

To increase our test coverage in general, and because I will add onto
this in the next commit to also test amcheck with incomplete splits.

This is copied from the similar test we had for GIN indexes. B-tree's
incomplete splits work similarly to GIN's, so with small changes, the
same test works for B-tree too.

Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://www.postgresql.org/message-id/abd65090-5336-42cc-b768-2bdd66738404@iki.fi