pgsql: Add regression test for short varlenas saved in TOAST relations

Add regression test for short varlenas saved in TOAST relations

toast_save_datum() has for a very long time some code able to handle
short varlenas (values up to 126 bytes reduced to a 1-byte header),
converting such varlenas to an external on-disk TOAST pointer with the
value saved uncompressed in the secondary TOAST relation.

There was zero coverage for this code path.  This commit adds a test
able to exercise it, relying on two external attributes, one with a low
toast_tuple_target, so as it is possible to trigger the threshold for
the insertion of short varlenas into the TOAST relation.

Author: Nikhil Kumar Veldanda <veldanda.nikhilkumar17@gmail.com>
Co-authored-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/aJAl7-NvIk0kZByz@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/225ebfe30a1ae9fda74f3d8f98ea6fa511b60624

Modified Files
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src/test/regress/expected/strings.out | 34 ++++++++++++++++++++++++++++++++++
src/test/regress/sql/strings.sql      | 20 ++++++++++++++++++++
2 files changed, 54 insertions(+)