Make use of `cargo test`s scoped test output

By using `test_writer`, cargo can automatically scope the output
of the test to the relevant thread and will also only output it
if the test fails or is run with `--nocapture`.

swap/tests/testutils/mod.rs

@@ -321,6 +321,7 @@ where
 
     let _guard = tracing_subscriber::fmt()
         .with_env_filter("warn,swap=debug,monero_harness=debug,monero_rpc=info,bitcoin_harness=info,testcontainers=info")
+        .with_test_writer()
         .set_default();
 
     let env_config = C::get_config();