Adding unit test cases for AuthCodeReceiver

tests/test_authcode.py

@@ -0,0 +1,23 @@
+import unittest
+
+from oauth2cli.authcode import AuthCodeReceiver
+
+
+class TestAuthCodeReceiver(unittest.TestCase):
+    def test_setup_at_a_given_port_and_teardown(self):
+        port = 12345  # Assuming this port is available
+        with AuthCodeReceiver(port=port) as receiver:
+            self.assertEqual(port, receiver.get_port())
+
+    def test_setup_at_a_ephemeral_port_and_teardown(self):
+        port = 0
+        with AuthCodeReceiver(port=port) as receiver:
+            self.assertNotEqual(port, receiver.get_port())
+
+    def test_no_two_concurrent_receivers_can_listen_on_same_port(self):
+        port = 12345  # Assuming this port is available
+        with AuthCodeReceiver(port=port) as receiver:
+            with self.assertRaises(OSError):
+                with AuthCodeReceiver(port=port) as receiver2:
+                    pass
+