Fix failing tests, negating Duration::MIN may overflow

Negating `Duration::MIN` does overflow, which is expected behaviour due
to it's internal representation. This happens twice in the current test
suite, this skips this part of the test to fix the remaining unit tests.

src/naive/date.rs

@@ -2200,7 +2200,9 @@ mod tests {
             let lhs = NaiveDate::from_ymd(y1, m1, d1);
             let sum = ymd.map(|(y, m, d)| NaiveDate::from_ymd(y, m, d));
             assert_eq!(lhs.checked_add_signed(rhs), sum);
-            assert_eq!(lhs.checked_sub_signed(-rhs), sum);
+            if let Some(rhs) = rhs.checked_mul(-1) {
+                assert_eq!(lhs.checked_sub_signed(rhs), sum);
+            }
         }
 
         check((2014, 1, 1), Duration::ZERO, Some((2014, 1, 1)));

src/naive/datetime.rs

@@ -2311,7 +2311,9 @@ mod tests {
             let sum =
                 result.map(|(y, m, d, h, n, s)| NaiveDate::from_ymd(y, m, d).and_hms(h, n, s));
             assert_eq!(lhs.checked_add_signed(rhs), sum);
-            assert_eq!(lhs.checked_sub_signed(-rhs), sum);
+            if let Some(rhs) = rhs.checked_mul(-1) {
+                assert_eq!(lhs.checked_sub_signed(rhs), sum);
+            }
         }
 
         check(