add _unique_name tests

tests/test_cli.py

@@ -377,3 +377,20 @@ def test_perform_args_on_youtube(youtube):
     cli.main()
     youtube.assert_called()
     cli._perform_args_on_youtube.assert_called()
+
+
+@mock.patch("pytube.cli.os.path.exists", return_value=False)
+def test_unique_name(path_exists):
+    assert (
+        cli._unique_name("base", "subtype", "video", "target")
+        == "target/base_video_0.subtype"
+    )
+
+
+@mock.patch("pytube.cli.os.path.exists")
+def test_unique_name_counter(path_exists):
+    path_exists.side_effect = [True, False]
+    assert (
+        cli._unique_name("base", "subtype", "video", "target")
+        == "target/base_video_1.subtype"
+    )