pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1e4e5783e7d77adb283ae7fa1ac99b0910a041b2

Modified Files
--------------
src/backend/access/nbtree/nbtinsert.c              |   9 ++
src/test/modules/meson.build                       |   1 +
src/test/modules/nbtree/Makefile                   |  28 ++++
.../nbtree/expected/nbtree_incomplete_splits.out   | 179 +++++++++++++++++++++
src/test/modules/nbtree/meson.build                |  16 ++
.../nbtree/sql/nbtree_incomplete_splits.sql        | 134 +++++++++++++++
6 files changed, 367 insertions(+)