pgsql: Add regression tests for pg_stat_progress_copy.tuples_skipped.

Add regression tests for pg_stat_progress_copy.tuples_skipped.

This commit adds tests to verify that tuples_skipped in pg_stat_progress_copy
works as expected. While existing tests checked other fields, tuples_skipped
was previously untested.

This improves test coverage and ensures accurate tracking of skipped tuples.

Author: Jian He <jian(dot)universality(at)gmail(dot)com>
Reviewed-by: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Josef Šimánek <josef(dot)simanek(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxFazq-bfyhiO0KBojR=yOr84E25Rqf6mHB0Ow0KPidkKw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/91ecb5e0bc7f8373ec9ff7d188bfd0d45f13a333

Modified Files
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src/test/regress/expected/copy.out | 11 ++++++++---
src/test/regress/sql/copy.sql | 10 +++++++++-
2 files changed, 17 insertions(+), 4 deletions(-)