fix test

Author: pvdrz

tests/pass/shims/time-with-isolation.rs

@@ -1,11 +1,5 @@
 use std::time::{Duration, Instant};
 
-fn duration_sanity(diff: Duration) {
-    // On my laptop, I observed times around 15-40ms. Add 10x lee-way both ways.
-    assert!(diff.as_millis() > 1);
-    assert!(diff.as_millis() < 500);
-}
-
 fn test_sleep() {
     let before = Instant::now();
     std::thread::sleep(Duration::from_millis(100));
@@ -26,7 +20,9 @@ fn main() {
     let diff = now2.duration_since(now1);
     assert_eq!(now1 + diff, now2);
     assert_eq!(now2 - diff, now1);
-    duration_sanity(diff);
+    // The virtual clock is deterministic and I got 29us. This should be updated if
+    // `NANOSECONDS_PER_BASIC_BLOCK` changes.
+    assert_eq!(diff.as_micros(), 29);
 
     test_sleep();
 }