Dogfood Duration API in std::time tests

This expands time's test suite to use more and in more places the
range of methods and constants added to Duration in recent
proposals for the sake of testing more API surface area and
improving legibility.

library/std/src/lib.rs

@@ -255,6 +255,7 @@
 #![feature(doc_spotlight)]
 #![feature(dropck_eyepatch)]
 #![feature(duration_constants)]
+#![feature(duration_zero)]
 #![feature(exact_size_is_empty)]
 #![feature(exhaustive_patterns)]
 #![feature(extend_one)]