I can definitely say that participation in test networks sometimes gives a good profit. But you need to understand that not all the testnets will eventually be paid, as the team may not reach the final stage of the project or simply get greedy without paying the reward. But given the fact that it is not difficult to pass the testnet, you need to participate and perhaps the reward will find you.
this i true not all testnet paid the participant and not guarantee we can pass the the test
in the other hand testnet is very rare comparing bounties and airdrop and usually paid testnet has same with airdrop and bounties that limit the participant