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Add SQL test for TOAST value allocations on rewrite

Add SQL test for TOAST value allocations on rewrite

The SQL test added in this commit check a specific code path that had no
coverage until now. When a TOAST datum is rewritten, toast_save_datum()
has a dedicated path to make sure that a new value is allocated if it
does not exist on the TOAST table yet.

This test uses a trick with PLAIN and EXTERNAL storage, with a tuple
large enough to be toasted and small enough to fit on a page. It is
initially stored in plain more, and the rewrite forces the tuple to be
stored externally. The key point is that there is no value allocated
during the initial insert, and that there is one after the rewrite. A
second pattern checked is the reuse of the same value across rewrites,
using \gset.

A set of patches under discussion is messing up with this area of the
code, so this makes sure that such rewrite cases remain consistent
across the board.

Author: Nikhil Kumar Veldanda <veldanda.nikhilkumar17@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAFAfj_E+kw5P713S8_jZyVgQAGVFfzFiTUJPrgo-TTtJJoazQw@mail.gmail.com

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