Add test coverage for pg_current_logfile() function.

There has been no coverage at all up to now.  Given Thomas Kellerer's
recent report, I suspect this may fail on (some?) Windows machines,
but let's find out.

Discussion: https://postgr.es/m/412ae8da-76bb-640f-039a-f3513499e53d@gmx.net

src/bin/pg_ctl/t/004_logrotate.pl

@@ -3,7 +3,7 @@ use warnings;
 
 use PostgresNode;
 use TestLib;
-use Test::More tests => 4;
+use Test::More tests => 5;
 use Time::HiRes qw(usleep);
 
 # Set up node with logging collector
@@ -47,6 +47,10 @@ for (my $attempts = 0; $attempts < $max_attempts; $attempts++)
 
 like($first_logfile, qr/division by zero/, 'found expected log file content');
 
+# While we're at it, test pg_current_logfile() function
+is($node->safe_psql('postgres', "SELECT pg_current_logfile('stderr')"),
+	$lfname, 'pg_current_logfile() gives correct answer');
+
 # Sleep 2 seconds and ask for log rotation; this should result in
 # output into a different log file name.
 sleep(2);