pgsql: doc: mention libpq regression tests

doc: mention libpq regression tests

Reported-by: Ryo Matsumura

Discussion: /messages/by-id/TYCPR01MB11316B3FB56EE54D70BF0CEF6E8E4A@TYCPR01MB11316.jpnprd01.prod.outlook.com

Backpatch-through: 11

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/caeb4cf191fac0f189647a64e5d246d4f378a960

Modified Files
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doc/src/sgml/regress.sgml | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)