Fix logical replication TAP test to read publisher log correctly.

Fix logical replication TAP test to read publisher log correctly.

Commit 5f13999aa11 added a TAP test for GUC settings passed via the
CONNECTION string in logical replication, but the buildfarm member
sungazer reported test failures.

The test incorrectly used the subscriber's log file position as the
starting offset when reading the publisher's log. As a result, the test
failed to find the expected log message in the publisher's log and
erroneously reported a failure.

This commit fixes the test to use the publisher's own log file position
when reading the publisher's log.

Also, to avoid similar confusion in the future, this commit splits the single
$log_location variable into $log_location_pub and $log_location_sub,
clearly distinguishing publisher and subscriber log positions.

Backpatched to v15, where commit 5f13999aa11 introduced the test.

Per buildfarm member sungazer.
This issue was reported and diagnosed by Alexander Lakhin.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/966ec3d8-1b6f-4f57-ae59-fc7d55bc9a5a@gmail.com
Backpatch-through: 15