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/keydb.rs

@@ -846,7 +846,7 @@ mod tests {
 
         let body = b"0xAABBCCDDAABBCCDDAABBCCDDAABBCCDD = Test\n".to_vec();
         let result = src
-            .update(|_url| Ok(body.clone()), "http://example.invalid/keydb.zip")
+            .update(|_url| Ok(body.clone()), "http://example.test/keydb.zip")
             .expect("update must succeed with a good fetch");
 
         assert_eq!(result.path, target, "update must save to the source's path");