Tests: fixed session reuse tests in ssl_certificate.t with TLSv1.3.

Previously, session data was retrieved too early, before server passed
application data, which usually means NewSessionTicket is not yet sent.
The fix is to ask server for application data, then retrieve a session.

ssl_certificate.t

diff --git a/ssl_certificate.t b/ssl_certificate.t
index 2f3d548..e02fceb 100644
--- a/ssl_certificate.t
+++ b/ssl_certificate.t
@@ -180,7 +180,7 @@
 
 # session reuse
 
-my ($s, $ssl) = get_ssl_socket('default', 8080);
+my ($s, $ssl) = get('default', 8080);
 my $ses = Net::SSLeay::get_session($ssl);
 
 like(get('default', 8080, $ses), qr/default:r/, 'session reused');
@@ -201,7 +201,8 @@
 	Net::SSLeay::write($ssl, 'GET / HTTP/1.0' . CRLF . CRLF);
 	my $r = Net::SSLeay::read($ssl);
 	$s->close();
-	return $r;
+	return $r unless wantarray();
+	return ($s, $ssl);
 }
 
 sub cert {