Restore robustness of TAP tests that wait for postmaster restart.

Several TAP tests use poll_query_until() to wait for the postmaster
to restart.  They were checking to see if a trivial query
(e.g. "SELECT 1") succeeds.  However, that's problematic in the wake
of commit 11e9caff, because now that we feed said query to psql
via stdin, we risk IPC::Run whining about a SIGPIPE failure if psql
quits before reading the query.  Hence, we can't use a nonempty
query in cases where we need to wait for connection failures to
stop happening.

Per the precedent of commits c757a3da and 6d41dd04, we can pass
"undef" as the query in such cases to ensure that IPC::Run has
nothing to write.  However, then we have to say that the expected
output is empty, and this exposes a deficiency in poll_query_until:
if psql fails altogether and returns empty stdout, poll_query_until
will treat that as a success!  That's because, contrary to its
documentation, it makes no actual check for psql failure, looking
neither at the exit status nor at stderr.

To fix that, adjust poll_query_until to insist on empty stderr as
well as a stdout match.  (I experimented with checking exit status
instead, but it seems that psql often does exit(1) in cases that we
need to consider successes.  That might be something to fix someday,
but it would be a non-back-patchable behavior change.)

Back-patch to v10.  The test cases needing this exist only as far
back as v11, but it seems wise to keep poll_query_until's behavior
the same in v10, in case we back-patch another such test case in
future.  (9.6 does not currently need this change, because in that
branch poll_query_until can't be told to accept empty stdout as
a success case.)

Per assorted buildfarm failures, mostly on hoverfly.

Discussion: https://postgr.es/m/CAA4eK1+zM6L4QSA1XMvXY_qqWwdUmqkOS1+hWvL8QcYEBGA1Uw@mail.gmail.com

src/test/perl/PostgresNode.pm

@@ -2140,8 +2140,10 @@ sub poll_query_until
 
 		$stdout =~ s/\r\n/\n/g if $Config{osname} eq 'msys';
 		chomp($stdout);
+		$stderr =~ s/\r\n/\n/g if $Config{osname} eq 'msys';
+		chomp($stderr);
 
-		if ($stdout eq $expected)
+		if ($stdout eq $expected && $stderr eq '')
 		{
 			return 1;
 		}
@@ -2154,8 +2156,6 @@ sub poll_query_until
 
 	# The query result didn't change in 180 seconds. Give up. Print the
 	# output from the last attempt, hopefully that's useful for debugging.
-	$stderr =~ s/\r\n/\n/g if $Config{osname} eq 'msys';
-	chomp($stderr);
 	diag qq(poll_query_until timed out executing this query:
 $query
 expecting this output:

src/test/recovery/t/013_crash_restart.pl

@@ -136,12 +136,8 @@ ok( pump_until(
 $monitor->finish;
 
 # Wait till server restarts
-is( $node->poll_query_until(
-		'postgres',
-		'SELECT $$restarted after sigquit$$;',
-		'restarted after sigquit'),
-	"1",
-	"reconnected after SIGQUIT");
+is($node->poll_query_until('postgres', undef, ''),
+	"1", "reconnected after SIGQUIT");
 
 
 # restart psql processes, now that the crash cycle finished
@@ -216,7 +212,7 @@ ok( pump_until(
 $monitor->finish;
 
 # Wait till server restarts
-is($node->poll_query_until('postgres', 'SELECT 1', '1'),
+is($node->poll_query_until('postgres', undef, ''),
 	"1", "reconnected after SIGKILL");
 
 # Make sure the committed rows survived, in-progress ones not

src/test/recovery/t/022_crash_temp_files.pl

@@ -139,7 +139,7 @@ $killme->finish;
 $killme2->finish;
 
 # Wait till server restarts
-$node->poll_query_until('postgres', 'SELECT 1', '1');
+$node->poll_query_until('postgres', undef, '');
 
 # Check for temporary files
 is( $node->safe_psql(
@@ -228,7 +228,7 @@ $killme->finish;
 $killme2->finish;
 
 # Wait till server restarts
-$node->poll_query_until('postgres', 'SELECT 1', '1');
+$node->poll_query_until('postgres', undef, '');
 
 # Check for temporary files -- should be there
 is( $node->safe_psql(