Tests: adjusted mail_smtp.t "long pipelined" TODO.

Prior to fixed reading with fully filled buffer in 2851e4c7de03 (1.21.0),
test for long pipelined commands leads to connection close by nginx, which
can result in RST and no response seen by the client, notably on win32.

While here, cleaned up permitted smtp_auth methods in that particular server.
diff --git a/mail_smtp.t b/mail_smtp.t
index 49fdbb8..c398d49 100644
--- a/mail_smtp.t
+++ b/mail_smtp.t
@@ -52,7 +52,7 @@
     server {
         protocol   smtp;
-        smtp_auth  login plain none cram-md5 external;
+        smtp_auth  none;
         smtp_client_buffer 128;
@@ -285,11 +285,10 @@
 	. 'RCPT TO:<foobar@example.com>' . CRLF
 	. 'RSET');
-$s->ok('long pipelined mail from');
 TODO: {
-local $TODO = 'not yet' unless $t->has_version('1.21.0');
+todo_skip 'long pipelined - not yet', 6 unless $t->has_version('1.21.0');
+$s->ok('long pipelined mail from');
 $s->ok('long pipelined rcpt to');
 $s->ok('long pipelined rcpt to 2');
 $s->ok('long pipelined rcpt to 3');