accounts/abi/bind/backends: fix race condition in simulated backend (ethereum#23898)

Now that `SimulatedBackend.SuggestGasPrice` inspects member values, a lock needs to be added to prevent a race condition.

Author: joshuacolvin0

accounts/abi/bind/backends/simulated.go

@@ -462,6 +462,9 @@ func (b *SimulatedBackend) PendingNonceAt(ctx context.Context, account common.Ad
 // SuggestGasPrice implements ContractTransactor.SuggestGasPrice. Since the simulated
 // chain doesn't have miners, we just return a gas price of 1 for any call.
 func (b *SimulatedBackend) SuggestGasPrice(ctx context.Context) (*big.Int, error) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
 	if b.pendingBlock.Header().BaseFee != nil {
 		return b.pendingBlock.Header().BaseFee, nil
 	}