Test for time.h constants

cbindgen doesn't properly export constants of type time_t. Add a test that fails if CLOCKS_PER_SEC is missing. (Also test for NULL.)

tests/Makefile

@@ -77,6 +77,7 @@ EXPECT_NAMES=\
 	strings \
 	sys_mman \
 	time/asctime \
+	time/constants \
 	time/gmtime \
 	time/localtime \
 	time/macros \