unit-testing firebase with swift fails

I have a basic unit-test for firebase with swift, but i get the following error:

Failed to get default FIRDatabase instance. Must call FIRApp.configure() before using FIRDatabase.

That happens although i call FIRApp.configure() the line before and it passes without throwing any error.

When i put the code in the AppDelegate it runs without any problems.

Please advise...

I am using xcode 7.3.1 my unit-test code