Tests: added proxy_timeout in mail tests. Default proxy_timeout in mail proxy is 24h, so if nginx shutdown after a test waits for the timeout to expire for some reason, this may take a while and hard to diagnose.

--- a/mail_resolver.t
+++ b/mail_resolver.t
@@ -46,6 +46,8 @@ mail {
     smtp_auth    none;
     server_name  locahost;
 
+    proxy_timeout 15s;
+
     # prevent useless resend
     resolver_timeout 2s;