test(lua): add example for testing is_descendant
test(lua): add example for testing is_descendant

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

htzh/lean,leanprover/lean,Kha/lean,javra/lean,leanprover-community/lean,soonhokong/lean-windows,soonhokong/travis_test,javra/lean,c-cube/lean,c-cube/lean,fpvandoorn/lean,leanprover/lean,dselsam/lean,fpvandoorn/lean2,avigad/lean,Kha/lean,levnach/lean,dselsam/lean,htzh/lean,soonhokong/lean,leodemoura/lean,fpvandoorn/lean2,eigengrau/lean,avigad/lean,digama0/lean,UlrikBuchholtz/lean,digama0/lean,rlewis1988/lean,johoelzl/lean,Kha/lean,fpvandoorn/lean,levnach/lean,c-cube/lean,javra/lean,soonhokong/lean-windows,c-cube/lean,sp3ctum/lean,johoelzl/lean,Kha/lean,levnach/lean,fgdorais/lean,leanprover/lean,c-cube/lean,soonhokong/lean-osx,eigengrau/lean,levnach/lean,fpvandoorn/lean,digama0/lean,leanprover/lean,javra/lean,leanprover-community/lean,eigengrau/lean,UlrikBuchholtz/lean,rlewis1988/lean,johoelzl/lean,eigengrau/lean,leanprover-community/lean,fpvandoorn/lean2,soonhokong/lean-windows,Kha/lean,soonhokong/lean-windows,avigad/lean,soonhokong/lean,rlewis1988/lean,johoelzl/lean,fgdorais/lean,javra/lean,leodemoura/lean,leodemoura/lean,digama0/lean,avigad/lean,leanprover/lean,soonhokong/lean-osx,sp3ctum/lean,fgdorais/lean,Kha/lean,avigad/lean,johoelzl/lean,leodemoura/lean,UlrikBuchholtz/lean,leodemoura/lean,sp3ctum/lean,soonhokong/travis_test,leodemoura/lean,fgdorais/lean,htzh/lean,digama0/lean,avigad/lean,rlewis1988/lean,leanprover/lean,dselsam/lean,soonhokong/lean-windows,sp3ctum/lean,eigengrau/lean,johoelzl/lean,leanprover-community/lean,levnach/lean,htzh/lean,fgdorais/lean,soonhokong/travis_test,UlrikBuchholtz/lean,digama0/lean,sp3ctum/lean,soonhokong/lean-osx,soonhokong/lean-osx,soonhokong/lean,leanprover-community/lean,rlewis1988/lean,dselsam/lean,fpvandoorn/lean2,soonhokong/travis_test,UlrikBuchholtz/lean,fpvandoorn/lean,dselsam/lean,fgdorais/lean,fpvandoorn/lean,leanprover-community/lean,soonhokong/lean-osx,soonhokong/lean,dselsam/lean,rlewis1988/lean,htzh/lean,soonhokong/lean,fpvandoorn/lean,fpvandoorn/lean2