Improve unit-tests

[tvdijen]

The unit tests currently only cover the 'happy flow' to ensure our Traits do what they need to do, but they should also cover the 'crappy flow' where we validate that invalid data is rejected.

An example for this would be something like this:

https://github.com/simplesamlphp/assert/blob/master/tests/Assert/NMTokensTest.php