Addition of %b/backend_type in log_line_prefix of TAP test logs

Hi all,

While navigating through the logs in the CI, the buildfarm, or even my
own machine, I've found myself spending a lot of time looking at
very specific log entries to find out the PID of a specific process,
sometimes mistaking a normal backend vs a logical WAL sender while
looking for a PID, or just looking for an auxiliary process.

I'd like to suggest the following change in Cluster.pm:
-   print $conf "log_line_prefix = '%m [%p] %q%a '\n";
+   print $conf "log_line_prefix = '%m [%b,%p] %q%a '\n";

It is then possible to match a backend_type with a PID.  That
increases the quantity of the logs, of course, but I'm finding that
really helpful to have.  But perhaps it's only me?

Thanks,
--
Michael