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.universality@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Josef Šimánek <josef.simanek@gmail.com>
Discussion: https://postgr.es/m/CACJufxFazq-bfyhiO0KBojR=yOr84E25Rqf6mHB0Ow0KPidkKw@mail.gmail.com