Improve handling and logging of TAP tests for pg_upgrade

Improve handling and logging of TAP tests for pg_upgrade

This commit includes a set of improvements to help with the debugging of
failures in these new TAP tests:
- Instead of a plain diff command to compare the dumps generated, use
File::Compare::compare for the same effect.  diff is still used to
provide more context in the event of an error.
- Log the contents of regression.diffs if the pg_regress command fails.
- Unify the format of the logs generated, getting inspiration from the
style used in 027_stream_regress.pl.

wrasse is the only buildfarm member that has reported a failure until
now after the introduction of 322becb, complaining that the dumps
generated do not match, and I am lacking information to understand what
is going in this environment.