BUG: util.testing.makeCustomIndex results were not sorted properly

mkdf(100,2) was returning
[(#_l0_g0, #_l1_g0), (#_l0_g1, #_l1_g1), (#_l0_g10, #_l1_g10), (#_l0_g100, #_l1_g100)

rather then:

[(#_l0_g0, #_l1_g0), (#_l0_g1, #_l1_g1), (#_l0_g2, #_l1_g2), (#_l0_g3, #_l1_g3)