tests/tcg: build tests with -Werror

Hopefully this will guard against sloppy code getting into our tests.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201210190417.31673-9-alex.bennee@linaro.org>

tests/tcg/Makefile.target

@@ -94,7 +94,7 @@ ifdef CONFIG_USER_ONLY
 -include $(SRC_PATH)/tests/tcg/$(TARGET_NAME)/Makefile.target
 
 # Add the common build options
-CFLAGS+=-Wall -O0 -g -fno-strict-aliasing
+CFLAGS+=-Wall -Werror -O0 -g -fno-strict-aliasing
 ifeq ($(BUILD_STATIC),y)
 LDFLAGS+=-static
 endif