eth/gasprice: ensure cache purging goroutine terminates with subscription (#31025)

eth/gasprice/gasprice.go

@@ -120,16 +120,23 @@ func NewOracle(backend OracleBackend, params Config, startPrice *big.Int) *Oracl
 
 	cache := lru.NewCache[cacheKey, processedFees](2048)
 	headEvent := make(chan core.ChainHeadEvent, 1)
-	backend.SubscribeChainHeadEvent(headEvent)
+	sub := backend.SubscribeChainHeadEvent(headEvent)
+	if sub != nil { // the gasprice testBackend doesn't support subscribing to head events
 		go func() {
 			var lastHead common.Hash
-		for ev := range headEvent {
+			for {
+				select {
+				case ev := <-headEvent:
 					if ev.Header.ParentHash != lastHead {
 						cache.Purge()
 					}
 					lastHead = ev.Header.Hash()
+				case <-sub.Err():
+					return
+				}
 			}
 		}()
+	}
 
 	return &Oracle{
 		backend:          backend,