Increase Duration approximate equal threshold to 1us

Previously this threshold when testing was 100ns, but the Windows
documentation states:

> which is a high resolution (<1us) time stamp

which presumably means that we could have up to 1us resolution, which
means that 100ns doesn't capture "equivalent" time intervals due to
various bits of rounding here and there.

It's hoped that this..

Closes rust-lang#56034

src/libstd/time.rs

@@ -481,7 +481,8 @@ mod tests {
             let (a, b) = ($a, $b);
             if a != b {
                 let (a, b) = if a > b {(a, b)} else {(b, a)};
-                assert!(a - Duration::new(0, 100) <= b);
+                assert!(a - Duration::new(0, 1000) <= b,
+                        "{:?} is not almost equal to {:?}", a, b);
             }
         })
     }