Add tests for presence of `RelayState` and `return` parameters sent to the SP

[baszoetekouw]

Add functional tests for the presence of the RelayState and return parameters in the SAML POST to the SP.

The tests are currently not present so if the RelayState parameter get lost somewhere in the login flow, this is not detected.

[baszoetekouw]

Note: this originated from an old ticket. Check if these tests have been implemented in the mean time,