remove supervisor-is-done upon test completion

src/agent/onefuzz-supervisor/src/agent/tests.rs

@@ -221,4 +221,10 @@ async fn test_emitted_state_failed_setup() {
     let coordinator: &CoordinatorDouble = agent.coordinator.downcast_ref().unwrap();
     let events = &coordinator.events;
     assert_eq!(events, &expected_events);
+
+    // TODO: at some point, the underlying tests should be updated to not write
+    // this file in the first place.
+    tokio::fs::remove_file(crate::done::done_path().unwrap())
+        .await
+        .unwrap();
 }