Fix: test corpus_inaccessible panic (#10019)

If system uptime is less than the duration in the test, thread
will panic due to: 'overflow when subtracting duration from instant'.

Changed duration to 20 seconds to make it unlikely the test will
fail due to this condition.

Since no operations that carry a similar risk of panic occur outside
of the tests, it doesn't seem warranted to depend on an external crate
to fix this.

ethcore/light/src/cache.rs

@@ -179,14 +179,15 @@ mod tests {
 
 	#[test]
 	fn corpus_inaccessible() {
-		let mut cache = Cache::new(Default::default(), Duration::from_secs(5 * 3600));
+		let duration = Duration::from_secs(20);
+		let mut cache = Cache::new(Default::default(), duration.clone());
 
 		cache.set_gas_price_corpus(vec![].into());
 		assert_eq!(cache.gas_price_corpus(), Some(vec![].into()));
 
 		{
 			let corpus_time = &mut cache.corpus.as_mut().unwrap().1;
-			*corpus_time = *corpus_time - Duration::from_secs(5 * 3600);
+			*corpus_time = *corpus_time - duration;
 		}
 		assert!(cache.gas_price_corpus().is_none());
 	}