Use -Werror in hardware/libhardware/tests

* Remove unused local variables.
* Suppress warning of unused template functions.
* Fix error of unused expression value.

Bug: 66996870
Test: build with WITH_TIDY=1
Change-Id: I5a37c24f3be0f61b0ae4552e34b7311f561d499e
diff --git a/tests/vehicle/Android.mk b/tests/vehicle/Android.mk
index eb41725..ab412fb 100644
--- a/tests/vehicle/Android.mk
+++ b/tests/vehicle/Android.mk
@@ -25,7 +25,7 @@
     liblog \
     libhardware \
 
-LOCAL_CFLAGS += -Wall -Wextra
+LOCAL_CFLAGS += -Wall -Wextra -Werror
 
 LOCAL_MODULE:= vehicle_tests
 LOCAL_MODULE_TAGS := tests