tests: fix potential crash
tests: fix potential crash

The idle source is not always removed and may crash next tests

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@gmail.com>

libvirt/libvirt-glib,libvirt/libvirt-glib,libvirt/libvirt-glib