tests: Trim newline from result returned by BackgroundPsql->query

tests: Trim newline from result returned by BackgroundPsql->query

This went unnoticed, because only a few existing callers of
BackgroundPsql->query used the result, and the ones that did were not
bothered by an extra newline. I noticed because I was about to add a
new test that checks the result.

Backport to all supported versions, since I just backported the
BackgroundPsql facility to all supported versions too.