Add tests for MostSeen decrement bug

This bug requires a specific set of circumstances to trigger
and become visible:
1. decrement a label that is non-zero but not first
2. repeat the decrement at least best_count()/2 times
3. increment that label
4. check the best count

backend/common/src/most_seen.rs

@@ -248,5 +248,11 @@ mod test {
         assert!(res.has_changed());
         assert_eq!(a.best_count(), 2);
         assert_eq!(*a.best(), "First"); // First is now ahead
+
+        a.remove(&"Third"); // 0, or 2 with bug #595
+        a.remove(&"Third"); // 0, or 4 with bug #595
+        a.insert(&"Third"); // 1, or 5 with bug #595
+        assert_eq!(a.best_count(), 2);
+        assert_eq!(*a.best(), "First"); // First is still ahead
     }
 }