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

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.

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