system test: add small delay for background transactions

Author: tsahee

system_tests/staker_test.go

@@ -46,6 +46,7 @@ import (
 
 func makeBackgroundTxs(ctx context.Context, builder *NodeBuilder) error {
 	for i := uint64(0); ctx.Err() == nil; i++ {
+		time.Sleep(time.Millisecond * 100)
 		builder.L2Info.Accounts["BackgroundUser"].Nonce.Store(i)
 		tx := builder.L2Info.PrepareTx("BackgroundUser", "BackgroundUser", builder.L2Info.TransferGas, common.Big0, nil)
 		err := builder.L2.Client.SendTransaction(ctx, tx)