fix: integer overflow in NNS tests in is_inactive (#1984)

In tests, `now` was observed to be set to something between 50 and 60
seconds. When `now` is set to less than 2 weeks, an integer overflow is
triggered, which can cause the neuron to be incorrectly recognized as
inactive. This happened during testing of `claim_neuron`.

This should not affect prod because in prod, `now` should always be much
larger than 2 weeks.

This was discovered using TLA codelink.

rs/nns/governance/src/neuron/types.rs

@@ -954,7 +954,8 @@ impl Neuron {
 
         // 3.2: Now, we know when self is "dissolved" (could be in the past, present, or future).
         // Thus, we can evaluate whether that happened sufficiently long ago.
-        let max_dissolved_at_timestamp_seconds_to_be_inactive = now - 2 * 7 * ONE_DAY_SECONDS;
+        let max_dissolved_at_timestamp_seconds_to_be_inactive =
+            now.saturating_sub(2 * 7 * ONE_DAY_SECONDS);
         if dissolved_at_timestamp_seconds > max_dissolved_at_timestamp_seconds_to_be_inactive {
             return false;
         }