Tests: SIGPIPE handling in mail tests. In contrast to http tests, mail tests generally do not try to handle SIGPIPE when writing to a socket, and instead rely on $SIG{PIPE} being set at the start of the test (see 96:ecff5407867c). Fixed some tests which don't do this.

--- a/mail_ssl.t
+++ b/mail_ssl.t
@@ -25,6 +25,8 @@ use Test::Nginx::SMTP;
 select STDERR; $| = 1;
 select STDOUT; $| = 1;
 
+local $SIG{PIPE} = 'IGNORE';
+
 eval {
 	require Net::SSLeay;
 	Net::SSLeay::load_error_strings();