eth/catalyst: ensure TxPool is synced in Fork (ethereum#29876)

This should fix an occasional test failure in ethclient/simulated.TestForkResendTx.
Inspection of logs revealed the cause of the failure to be that the txpool was not done
reorganizing by the time Fork is called.

eth/catalyst/simulated_beacon.go

@@ -279,9 +279,12 @@ func (c *SimulatedBeacon) Rollback() {
 
 // Fork sets the head to the provided hash.
 func (c *SimulatedBeacon) Fork(parentHash common.Hash) error {
+	// Ensure no pending transactions.
+	c.eth.TxPool().Sync()
 	if len(c.eth.TxPool().Pending(txpool.PendingFilter{})) != 0 {
 		return errors.New("pending block dirty")
 	}
+
 	parent := c.eth.BlockChain().GetBlockByHash(parentHash)
 	if parent == nil {
 		return errors.New("parent not found")