Makefile: force tests for both versions of eclipse

The dependencies v5-test and v7-test both depends on test but with
different environment variables; that's however not sufficent for
Make, which consider that once test is done, it is not necessary to
execute it again.

Makefile

-.PHONY: all bundle clean clean_bundle simplex simplex_epilog
+.PHONY: all bundle clean clean_bundle simplex simplex_epilog tests
 
 # NB. This makefile uses rules like v5-% and v7-% to setup
 # the required environment before executing the associated
@@ -152,7 +152,9 @@ archives:
 test:
 	./test.sh $(BUNDLE_ROOT)/colibri -j2
 
-tests: v5-test v7-test
+tests:
+	make v5-test
+	make v7-test
 
 # ==================================================
 # RELEASE