increase txorphan harness stability

initialize variable

src/txorphanage.cpp

@@ -119,7 +119,6 @@ void TxOrphanage::LimitOrphans(unsigned int max_orphans, FastRandomContext& rng)
     LOCK(m_mutex);
 
     unsigned int nEvicted = 0;
-    static NodeSeconds nNextSweep;
     auto nNow{Now<NodeSeconds>()};
     if (nNextSweep <= nNow) {
         // Sweep out expired orphan pool entries:

src/txorphanage.h

@@ -105,6 +105,9 @@ protected:
 
     /** Erase an orphan by wtxid */
     int EraseTxNoLock(const Wtxid& wtxid) EXCLUSIVE_LOCKS_REQUIRED(m_mutex);
+
+    /** Timestamp for the next scheduled sweep of expired orphans */
+    NodeSeconds nNextSweep GUARDED_BY(m_mutex){0s};
 };
 
 #endif // BITCOIN_TXORPHANAGE_H