TAP / recovery-test fs-level backups, psql enhancements etc

Hi all

I've been working with the new TAP tests for recovery and have a number of
enhancements I'd like to make to the tooling to make writing tests easier
and nicer. I've also included two improvements proposed by Kyotaro
HORIGUCHI in the prior thread about the now-committed TAP recovery tests.

I developed these changes as part of testing failover slots and logical
decoding timeline following, where I found a need for better control over
psql, the ability to make filesystem level backups, etc. It doesn't make
sense to try to jam all that into my test script when it belongs in the
infrastructure.

Patches attached, each explains what it does and what for.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services