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
daa99a9 (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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

Makefile

@@ -591,6 +591,7 @@ SCRIPT_PYTHON =
 SCRIPT_SH =
 SCRIPT_LIB =
 TEST_BUILTINS_OBJS =
+TEST_OBJS =
 TEST_PROGRAMS_NEED_X =
 THIRD_PARTY_SOURCES =