[Clippy] Swap instant_subtraction to use diagnostic item instead of…

… path

std/src/time.rs

@@ -280,6 +280,7 @@ impl Instant {
     /// ```
     #[must_use]
     #[stable(feature = "time2", since = "1.8.0")]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "instant_now")]
     pub fn now() -> Instant {
         Instant(time::Instant::now())
     }