Add back infer_vtree tests

[khosravipasha]

This was the removed tests. Currently cannot load logic circuit from psdd files, so would need to go around that (find a sdd to test this on or move the test to ProbCircuits.

    little = load_logic_circuit(zoo_psdd_file("little_4var.psdd")) 
    little_vtree = zoo_vtree("little_4var.vtree")

    inferred = infer_vtree(little)::Vtree
    @test respects_vtree(little, little_vtree)
    @test respects_vtree(little, inferred)