tests: Explicitly remove a timeout handler in gapplication test

Otherwise if, for whatever reason, the `app` loses its D-Bus name,
`g_application_quit()` is called from `name_was_lost()` before it’s
called from `quit_already()`, and then `quit_already()` does an invalid
read on `app`.

If the name was not meant to be lost at this point in the test, the
subsequent `g_assert_false (name_lost)` will catch that, so this change
shouldn’t cause the test to pass unnecessarily.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>