You can not test these compressor PTC relays, if the overload ( klixon ) is cutting out, then it is possible the relay is faulty or it could be the compressor itself, without a specilized hermetic compressor tester to check compressor, your only option is to buy a new relay and see if that resolves your issue.
Your compressor is an Aspera EMZ26CLC and I believe the relay is available on UK Whitegoods part shop as SKU#2260209040 but please check with them beforehand.