pg_upgrade: Test --check with a running source server

Hi hackers,

While doing some tests for [0], I noticed that we don't have coverage for
pg_upgrade --check with a running source server.

Indeed, commit 4fff78f009 added TAP coverage for pg_upgrade --check, but only
after stopping the source server.

0001 attached adds coverage with the source still running. This covers live server
detection, connection details from postmaster.pid, control data from pg_controldata,
reuse of the running postmaster, and the requirement for different source and target
ports.

It also tests that logical slots with unconsumed WAL are accepted during a live
check. The existing offline check verifies that the same slots are rejected after
shutdown.

Remarks:

1/ my CI was failing on Windows (at the new 003_logical_slots.pl test) for the
same reason why a sleep is part of 001_start_stop.pl. So adding the same comment
and sleep in 003_logical_slots.pl too.

2/ while at 1/ I observed the comment in 001_start_stop.pl was using a stale
function name. 0002 fixes it.

3/ the proposed tests would have had catch === 2 mentioned in [0].

[0]: https://postgr.es/m/apUL3N4IE934qJ08%40bdtpg

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com