Tests: fixed spurious mail_imap_ssl.t failures after 408fe0dd3fed.

After 408fe0dd3fed, shutdown of nginx worker processes might happen before
the last auth_http request is accepted, leading to the "log - trusted cert"
test failure.  Fix is to restore $s->read() call removed in 408fe0dd3fed.

mail_imap_ssl.t

diff --git a/mail_imap_ssl.t b/mail_imap_ssl.t
index 4ba3bbe..145ac14 100644
--- a/mail_imap_ssl.t
+++ b/mail_imap_ssl.t
@@ -207,6 +207,7 @@
 );
 $s->ok('trusted cert');
 $s->send('1 AUTHENTICATE PLAIN ' . $cred->("s5"));
+$s->read();
 undef $s;
 
 # test auth_http request header fields with access_log