Tests: marked "requests - responses" proxy_requests test as unsafe.

stream_udp_proxy_requests.t

diff --git a/stream_udp_proxy_requests.t b/stream_udp_proxy_requests.t
index 2ea0fbe..242e5cd 100644
--- a/stream_udp_proxy_requests.t
+++ b/stream_udp_proxy_requests.t
@@ -159,8 +159,14 @@
 $s = dgram('127.0.0.1:' . port(8985));
 $s->write('1') for 1 .. 5;
 $b = join ' ', map { $s->read() } (1 .. 10);
+
+SKIP: {
+skip 'session could early terminate', 1 unless $ENV{TEST_NGINX_UNSAFE};
+
 like($b, qr/^(\d+ 1) \1 (?!\1)(\d+ 1) \2 (?!\2)\d+ 1$/, 'requests - responses');
 
+}
+
 $t->stop();
 
 is($t->read_file('s.log'), <<EOF, 'uneven session status - slow backend');