test: Use mockable time in GuessVerificationProgress

Also add a test.

src/validation.cpp

@@ -5572,7 +5572,7 @@ double ChainstateManager::GuessVerificationProgress(const CBlockIndex* pindex) c
         return 0.0;
     }
 
-    int64_t nNow = time(nullptr);
+    const int64_t nNow{TicksSinceEpoch<std::chrono::seconds>(NodeClock::now())};
 
     double fTxTotal;