Reimplement interactive tests without gtk_main()

gtk_main() is gone in GTK 4, but the tests in the tests/interactive/ directory currently rely on it.

GTK 4 removes the gtk_main_* family of APIs. The recommended replacement is GtkApplication, but you can also iterate the GLib main loop directly, using GMainContext APIs. The replacement for gtk_events_pending() is g_main_context_pending(), the replacement for gtk_main_iteration() is g_main_context_iteration().

In GTK 4, you can use this replacement that will iterate the default main loop until all windows have been closed:

while (g_list_model_get_n_items (gtk_window_get_toplevels ()) > 0)
  g_main_context_iteration (NULL, TRUE);

In order to port to GTK 4, we must either solve this issue soon or disable the interactive tests and fix this later.