Tests: adjusted worker_shutdown_timeout_h2.t shutdown delay more.

A signal is made sure to be sent after nginx processed HEADERS,
while not unnecessarily delaying it.

worker_shutdown_timeout_h2.t

diff --git a/worker_shutdown_timeout_h2.t b/worker_shutdown_timeout_h2.t
index 4307f11..7265dd8 100644
--- a/worker_shutdown_timeout_h2.t
+++ b/worker_shutdown_timeout_h2.t
@@ -58,7 +58,9 @@
 my $s = Test::Nginx::HTTP2->new();
 ok($s->new_stream(), 'new stream');
 
-select undef, undef, undef, 0.2;
+$s->h2_ping('SEE-THIS');
+$s->read(all => [{ type => 'PING' }]);
+
 $t->stop();
 
 TODO: {