Check status of nodes after regression test run in 027_stream_regress

Check status of nodes after regression test run in 027_stream_regress

This commit improves the recovery TAP test 027_stream_regress so as
regression diffs are printed only if both the primary and the standby
are still alive after the main regression test suite finishes, relying
on d4c9195eff41 to do the job.

Particularly, a crash of the primary could scribble the contents
reported with mostly useless data, as the diffs would refer to query
that failed to run, not necessarily the cause of the crash.

Suggested-by: Andres Freund <andres@anarazel.de>
Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAN55FZ1D6KXvjSs7YGsDeadqCxNF3UUhjRAfforzzP0k-cE=bA@mail.gmail.com