pgsql: Improve regression test coverage of GiST index building.

Improve regression test coverage of GiST index building.

Add a test case that exercises the "buffering build" code path.
This covers almost all the non-error-case lines in gistbuild.c
and gistbuildbuffers.c.

Matheus Alcantara, based on earlier work by Pavel Borisov

Discussion: /messages/by-id/3z8Fde-IHbW57a7bEZtaf19f4YOCWu67IZoWJoGW18rKD9R16ZHHchf4d7KFI3Yg7-0N4NonFuwKEgh98HjMCZYoVx7KOioPo6Wn2nZRpf4=@pm.me

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6a1f082abac9da756d473e16238a906ca5a592dc

Modified Files
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src/test/regress/expected/gist.out | 3 +++
src/test/regress/sql/gist.sql | 4 ++++
2 files changed, 7 insertions(+)