pgsql: Add more tests with clause STORAGE on table and TOAST interactio

Add more tests with clause STORAGE on table and TOAST interactions

This commit adds more tests to cover STORAGE MAIN and EXTENDED, checking
how these use TOAST tables.  EXTENDED is already widely tested as the
default behavior, but there were no tests where the clause pattern is
directly specified.  STORAGE MAIN and its interactions with TOAST was
not covered at all.

This hole in the tests has been noticed for STORAGE MAIN (inline
compressible varlenas), where I have managed to break the backend
without the tests able to notice the breakage while playing with the
varlena structures.

Reviewed-by: Nikhil Kumar Veldanda <veldanda.nikhilkumar17@gmail.com>
Discussion: https://postgr.es/m/aXMdX1UTHnzYPkHk@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/168765e5d42be7d3ef750e9e292e14ef94b489e1

Modified Files
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src/test/regress/expected/strings.out | 54 +++++++++++++++++++++++++++++++++++
src/test/regress/sql/strings.sql      | 24 ++++++++++++++++
2 files changed, 78 insertions(+)