pyportaltest: Allow configurable mainloop timeouts

For now we keep the mainloop property with a default timeout of 2000ms
but future tests can make this more specific.

tests/pyportaltest/__init__.py

@@ -143,6 +143,36 @@ def tearDown(self):
             self.dbus_monitor.terminate()
             self.dbus_monitor.wait()
 
+    def setup_mainloop_with_timeout(self, timeout_ms: int):
+        """
+        Set our mainloop for this test. This mainloop automatically quits after
+        the given timeout (if the timeout is nonzero), but only on the first
+        call to run():
+
+        >>> loop = self.setup_mainloop_with_timeout(timeout_ms=1000)
+        >>> loop.run()  # exits after 1000ms
+        >>> loop.run()  # loops forever now
+        >>> loop2 = self.setup_mainloop_with_timeout(timeout_ms=2000)
+        >>> loop2.run()  # exits after 2000ms
+        >>> assert loop == loop2
+
+        That's usually enough for tests, if you need to call mainloop.run()
+        repeatedly ensure that a timeout handler is set to ensure quick test
+        case failure in case of error.
+        """
+        if self._mainloop is None:
+
+            self._mainloop = GLib.MainLoop()
+
+        def quit():
+            self._mainloop.quit()
+            self._mainloop = None
+
+        if timeout_ms:
+            GLib.timeout_add(timeout_ms, quit)
+
+        return self._mainloop
+
     @property
     def mainloop(self):
         """
@@ -152,16 +182,7 @@ def mainloop(self):
         timeout handler is set to ensure quick test case failure  in case of
         error.
         """
-        if self._mainloop is None:
-
-            def quit():
-                self._mainloop.quit()
-                self._mainloop = None
-
-            self._mainloop = GLib.MainLoop()
-            GLib.timeout_add(2000, quit)
-
-        return self._mainloop
+        return self.setup_mainloop_with_timeout(timeout_ms=2000)
 
     def assert_version_eq(self, version: int):
         """Assert the given version number is the one our portal exports"""

tests/pyportaltest/test_remotedesktop.py

@@ -39,8 +39,7 @@ def short_mainloop(self):
         Run a short iteration of our mainloop. Useful to for pure method calls
         that still require dbusmock etc. to update after the invocation.
         """
-        GLib.timeout_add(200, self.mainloop.quit)
-        self.mainloop.run()
+        self.setup_mainloop_with_timeout(timeout_ms=200).run()
 
     def create_session(
         self,