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.

src/test/perl/PostgreSQL/Test/BackgroundPsql.pm

@@ -223,7 +223,7 @@ sub query
 	$output = $self->{stdout};
 
 	# remove banner again, our caller doesn't care
-	$output =~ s/\n$banner$//s;
+	$output =~ s/\n$banner\n$//s;
 
 	# clear out output for the next query
 	$self->{stdout} = '';