Makefile: guard against TEST_OBJS in the environment

Add TEST_OBJS to the list of other *_OBJS variables we reset. We had already established this pattern when TEST_OBJS was introduced in daa99a91729 (Makefile: make sure test helpers are rebuilt when headers change, 2010-01-26), but it wasn't added to the list in that commit along with the rest. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>