update tests to use new Pester 4 syntax

[JamesWTruher]

after #882 is merged, we should probably go over the tests again and follow the new syntax format as described here: https://github.com/pester/Pester/wiki. There's no urgency for this as Pester 4 is mostly backward compatible with Pester 3, but inconsistent syntax may cause issues over time with regard to readability and maintenance.