Two "make check" fixes for late changes to the thread-context patch

Two "make check" fixes for late changes to the thread-context patch

g_simple_async_result_complete() now checks that it's being run from
the correct main loop, so tests/gio/simple-async-result was failing,
because it called it from outside any main loop. (And gio's pltcheck
was failing because I hadn't added g_main_current_source() to it.)