test: don't assert the value returned by relay_fee

Changed `test_relay_fee` to no longer assert the value of min
relay fee, and instead assert that the value is non-zero.
This fixes a test failure likely due to some nodes now having a
smaller default min relay fee (100sat).

src/raw_client.rs

@@ -1249,7 +1249,7 @@ mod test {
         let client = RawClient::new(get_test_server(), None).unwrap();
 
         let resp = client.relay_fee().unwrap();
-        assert_eq!(resp, 0.00001);
+        assert!(resp > 0.0);
     }
 
     #[test]