Make sure liberr-testing is build with testing flags.

src/lib/err/include.am

@@ -11,6 +11,8 @@ src_lib_libtor_err_a_SOURCES =			\
 
 src_lib_libtor_err_testing_a_SOURCES = \
 	$(src_lib_libtor_err_a_SOURCES)
+src_lib_libtor_err_testing_a_CPPFLAGS = $(AM_CPPFLAGS) $(TEST_CPPFLAGS)
+src_lib_libtor_err_testing_a_CFLAGS = $(AM_CFLAGS) $(TEST_CFLAGS)
 
 noinst_HEADERS +=					\
 	src/lib/err/backtrace.h				\