Remove mutable ret pattern and test feature combinations

After adding test for feature combinations, I found a corner case where, when Transport is dropped and the process is terminated in a test, the `close` Python future is not awaited.
I don't know what other situations this issue may arise, so I have safe-guarded it via `block_on` instead of spawning a thread.

rust/src/wrapper/transport.rs

@@ -15,6 +15,7 @@
 //! HCI packet transport
 
 use crate::wrapper::controller::Controller;
+use futures::executor::block_on;
 use pyo3::{intern, types::PyModule, PyObject, PyResult, Python};
 
 /// A source/sink pair for HCI packet I/O.
@@ -58,9 +59,9 @@ impl Transport {
 
 impl Drop for Transport {
     fn drop(&mut self) {
-        // can't await in a Drop impl, but we can at least spawn a task to do it
-        let obj = self.0.clone();
-        tokio::spawn(async move { Self(obj).close().await });
+        // don't spawn a thread to handle closing, as it may get dropped at program termination,
+        // resulting in `RuntimeWarning: coroutine ... was never awaited` from Python
+        let _ = block_on(self.close());
     }
 }