Use -Werror in bionic/tests

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I64a0458c787054a0694c736be20dc53772ff3781
diff --git a/tests/libs/Android.mk b/tests/libs/Android.mk
index a54318d..5bd028d 100644
--- a/tests/libs/Android.mk
+++ b/tests/libs/Android.mk
@@ -17,7 +17,7 @@
 LOCAL_PATH := $(call my-dir)
 TEST_PATH := $(LOCAL_PATH)/..
 
-common_cppflags :=
+common_cppflags := -Wall -Werror
 common_additional_dependencies := \
     $(LOCAL_PATH)/Android.mk \
     $(LOCAL_PATH)/Android.build.dt_runpath.mk \