Testing the logic in PowerShell Parameter Validation Attributes doesn't work at the moment. The framework depends on the Exception class to be declared public instead of internal.
I reported this issue to the PowerShell group at connect.mircosoft.com
Workaround 1: Wrap the parameter validation attribute exceptions into an System.InvalidArgumentException. E.g. declare helper function that does the validation and call this helper function within a try catch block.
There needs to be a feature change to handle exceptions that are declared internal.