Increase timeout in multixid_conversion upgrade test

Increase timeout in multixid_conversion upgrade test

The workload to generate multixids before upgrade is very slow on
buildfarm members running with JIT enabled. The workload runs a lot of
small queries, so it's unsurprising that JIT makes it slower. On my
laptop it nevertheless runs in under 10 s even with JIT enabled, while
some buildfarm members have been hitting the 180 s timeout. That seems
extreme, but I suppose it's still expected on very slow and busy
buildfarm animals. The timeout applies to the BackgroundPsql sessions
as whole rather than the individual queries.

Bump up the timeout to avoid the test failures. Add periodic progress
reports to the test output so that we get a better picture of just how
slow the test is.

In the passing, also fix comments about how many multixids and members
the workload generates. The comments were written based on 10 parallel
connections, but it actually uses 20.

Discussion: https://www.postgresql.org/message-id/b7faf07c-7d2c-4f35-8c43-392e057153ef@gmail.com