antispam test: run it serially, like the tests sharing its port

It listens on HTTP 19048, as the dkim2 DSN and report tests do. Those are
marked serial, this wasn't, so when the SMTP tests ran together (as
upstream's CI runs them) it could start beside them and requests reached
whichever server had the port: missing JMAP creates here, and 'You must
authenticate first' in dkim2_dsn_is_signed.

tests/src/smtp/inbound/antispam.rs

@@ -68,6 +68,10 @@ use std::{
     time::{Duration, Instant},
 };
 
+// inbuxa: its HTTP listener (19048) is shared with the dkim2 and report
+// tests, which are serial; without this it ran beside them and each got the
+// other's server.
+#[serial_test::serial]
 #[tokio::test(flavor = "multi_thread")]
 async fn antispam() {
     let mut test = TestServerBuilder::new("smtp_antispam_test")