test: use example.test placeholder host, not .invalid

leak-guard bans the .invalid TLD (grouped with private .internal/.local/etc);
example.test is the RFC 6761 reserved test TLD — equally unreachable, not on
the ban list — so the leak-guard CI check passes.

src/online.rs

@@ -416,7 +416,7 @@ mod tests {
     /// it's the deliberate no-op stub.
     #[test]
     fn host_certs_is_noop_empty_no_network() {
-        let src = OnlineSource::new("http://example.invalid/keys", "secret");
+        let src = OnlineSource::new("http://example.test/keys", "secret");
         assert!(
             KeySource::host_certs(&src, None).is_empty(),
             "online host_certs must be an empty no-op (no network)"