pgsql: Add TAP test for GUC settings passed via CONNECTION in logical r

Add TAP test for GUC settings passed via CONNECTION in logical replication.

Commit d926462d819 restored the behavior of passing GUC settings from
the CONNECTION string to the publisher's walsender, allowing per-connection
configuration.

This commit adds a TAP test to verify that behavior works correctly.

Since commit d926462d819 was recently applied and backpatched to v15,
this follow-up commit is also backpatched accordingly.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Chao Li <lic(at)highgo(dot)com>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Reviewed-by: Japin Li <japinli(at)hotmail(dot)com>
Discussion: https://postgr.es/m/CAHGQGwGYV+-abbKwdrM2UHUe-JYOFWmsrs6=QicyJO-j+-Widw@mail.gmail.com
Backpatch-through: 15

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6ec5968151252b6c01f1150f668fa9331db1313e

Modified Files
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src/test/subscription/t/001_rep_changes.pl | 29 ++++++++++++++++++++++++++++-
1 file changed, 28 insertions(+), 1 deletion(-)