tests/dbus-runner: Detect nested invocations to skip mocking

The test and dist CI jobs run wrap the meson calls in dbus-runner to
avoid setting up dbus servers and mocking services for every test but
the dbus-runner invocation from meson test didn't actually skip all the
setup.

This nested mocking also doesn't work because the system bus is assumed
to be the host system bus and not a mocked one.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2618>

src/tests/mutter_dbusrunner.py

@@ -258,14 +258,15 @@ ret = logind_helpers.open_file_direct(major, minor)
                 return template_path
         raise FileNotFoundError(f'Couldnt find a {template_name} template')
 
-    def wrap_call(self, args):
+
+def wrap_call(args, wrapper):
     env = {}
     env.update(os.environ)
     env['NO_AT_BRIDGE'] = '1'
     env['GTK_A11Y'] = 'none'
     env['GSETTINGS_BACKEND'] = 'memory'
+    env['META_DBUS_RUNNER_ACTIVE'] = '1'
 
-        wrapper = env.get('META_DBUS_RUNNER_WRAPPER')
     if wrapper == 'gdb':
         args = ['gdb', '-ex', 'r', '-ex', 'bt full', '--args'] + args
     elif wrapper:
@@ -287,14 +288,27 @@ def meta_run(klass):
     if rest[0] == '--':
       rest.pop(0)
 
+    result = 1
+
+    if os.getenv('META_DBUS_RUNNER_ACTIVE') == None:
         klass.setUpClass(args.kvm, args.launch)
         runner = klass()
         runner.assertGreater(len(rest), 0)
-    result = 1
+        wrapper = os.getenv('META_DBUS_RUNNER_WRAPPER')
 
         try:
             print('Running test case...', file=sys.stderr)
-        result = runner.wrap_call(rest)
+            result = wrap_call(rest, wrapper)
         finally:
             MutterDBusRunner.tearDownClass()
+    else:
+        try:
+            print(('Inside a nested meta-dbus-runner: '
+                   'Not re-creating mocked environment.'),
+                  file=sys.stderr)
+            print('Running test case...', file=sys.stderr)
+            result = wrap_call(rest, None)
+        finally:
+            pass
+
     return result