tests: update test_fish.py

Add `test_fish_path` to verify that the PATH is reverted to the
original PATH after virtualenv deactivation. This verification
confirms that the `_OLD_VIRTUAL_PATH` was correctly set in fish during
the activation of the virtualenv.

Author: r5d

tests/unit/activation/test_fish.py

@@ -21,3 +21,33 @@ def print_prompt(self):
             return "fish_prompt"
 
     activation_tester(Fish)
+
+
+@pytest.mark.skipif(IS_WIN, reason="we have not setup fish in CI yet")
+def test_fish_path(activation_tester_class, activation_tester, monkeypatch, tmp_path):
+    monkeypatch.setenv("HOME", str(tmp_path))
+    fish_conf_dir = tmp_path / ".config" / "fish"
+    fish_conf_dir.mkdir(parents=True)
+    (fish_conf_dir / "config.fish").write_text("", encoding="utf-8")
+
+    class Fish(activation_tester_class):
+        def __init__(self, session) -> None:
+            super().__init__(FishActivator, session, "fish", "activate.fish", "fish")
+
+        def print_prompt(self):
+            return "fish_prompt"
+
+        def _get_test_lines(self, activate_script):
+            return [
+                self.print_os_env_var("PATH"),
+                self.activate_call(activate_script),
+                self.print_os_env_var("PATH"),
+                self.deactivate,
+                self.print_os_env_var("PATH"),
+            ]
+
+        def assert_output(self, out, raw, _):
+            assert out[0] == out[2], raw
+            assert out[0] != out[1], raw
+
+    activation_tester(Fish)