...Usually testnet airdrop needs some money for gas fee and other requirements to be considered as participant. This isn't the usual airdrop where participants do requirements such as spreading the project, promoted or getting referrals. Testnet airdrops also has less abusers since we need money for gas fee. Though not every testnet has a guaranteed reward, So be meticulous on choosing a project if you are aiming for that reward.
You have incorrect information about completing tasks in the test network. To pay for gas in any of the test networks, test tokens are used as payment. Accordingly, you do not incur any material expenses, except for the time spent on completing the testnet tasks.