tests: Use class methods for class scoped fixtures

Using class-scoped fixture as instance method is deprecated in pytest 9.1 and
support will be removed in 10.0.

See: https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method

Tests:

- Run functional tests for samba, matrixsynapse (unrelated failure), and
bepasty.

Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/matrixsynapse/tests/test_functional.py

@@ -17,7 +17,8 @@ class TestMatrixSynapseApp(functional.BaseAppTests):
     diagnostics_delay = 1
 
     @pytest.fixture(scope='class', autouse=True)
-    def fixture_setup(self, session_browser):
+    @classmethod
+    def fixture_setup(cls, session_browser):
         """Setup the app."""
         functional.login(session_browser)
         functional.domain_add(session_browser, 'mydomain.example')

plinth/modules/samba/tests/test_functional.py

@@ -21,7 +21,8 @@ class TestSambaApp(functional.BaseAppTests):
     has_web = False
 
     @pytest.fixture(scope='class', autouse=True)
-    def fixture_setup(self, session_browser):
+    @classmethod
+    def fixture_setup(cls, session_browser):
         """Setup the app."""
         functional.login(session_browser)
         functional.networks_set_firewall_zone(session_browser, 'internal')

plinth/tests/functional/__init__.py

@@ -837,12 +837,13 @@ class BaseAppTests:
         install(session_browser, self.app_name)
 
     @pytest.fixture(autouse=True, scope='class', name='disable_after_tests')
-    def fixture_disable_after_tests(self, session_browser):
+    @classmethod
+    def fixture_disable_after_tests(cls, session_browser):
         """Disable the app after running tests."""
         yield
-        if self.disable_after_tests and is_installed(session_browser,
-                                                     self.app_name):
-            app_disable(session_browser, self.app_name)
+        if cls.disable_after_tests and is_installed(session_browser,
+                                                    cls.app_name):
+            app_disable(session_browser, cls.app_name)
 
     @pytest.fixture(autouse=True, name='background')
     def fixture_background(self, session_browser, disable_after_tests):