feat: add domain models for test configuration and results

lib/domain/models/iperf3_result.dart

@@ -0,0 +1,10 @@
+/// Raw result from an iperf3 execution.
+class Iperf3Result {
+  final String rawOutput;
+  final int exitCode;
+
+  const Iperf3Result({
+    required this.rawOutput,
+    required this.exitCode,
+  });
+}