In order to perform test payments, you have to enter TEST=1 on your website or enable test payments in your CMS plugin.
In your Paysera account, go to Projects and Activities > My Projects, click on Project settings > Paysera Checkout settings. Next check the following boxes:
Allow test payments
Accept inflows from any informational system (query website address will not be checked).
After performing test payments, please enter TEST=0 in your system and uncheck the boxes. Your project does not have to be activated and confirmed by the Paysera system administrator to perform test payments.