New targets to compile and install rollback tests only with USE=rbtest emerge ...

Review URL: http://codereview.chromium.org/2962002
diff --git a/Makefile b/Makefile
index 638d512..8d78466 100644
--- a/Makefile
+++ b/Makefile
@@ -35,7 +35,9 @@
 install:
 	$(MAKE) -C utility install
 	$(MAKE) -C cgpt install
-	$(MAKE) -C tests install
 
 runtests:
 	$(MAKE) -C tests runtests
+
+rbtest:
+	$(MAKE) -C tests rbtest