for testing purposes I need to stress an 8-port managed ethernet switch. Essentially, I want to simulate a scenario in which two hosts on the switch are exchanging data and the other hosts connected to the switch are trying to use all the available bandwidth. The idea is to see how a network application behaves when the other hosts on the same switch are using the network too much. How would you do that? Thanks a lot in advance!
Purpose-built Ethernet test gear is highly specialized and can be very expensive. You might be able to find some used gear that does what you want for 100Base-TX applications, but I doubt you're going to find gigabit gear for anything you want to spend.
As the comments have already stated, most commodity gigabit Ethernet silicon today is non-blocking and you can take the manufacturer at their word that the gear is going to do what it says. (Pretty much everybody, except for the big "name brand" switch manufacturers, are just using ASICs from the same OEMs anyway).
Having said that, there's no reason that you can't do as @dusan.bajic says in the comments and use a traffic generator tool (