Unit tests do not work with Pester 5.x

[lupino3]

It looks like unit tests don't work with Pester 5.x.

I was able to reproduce locally the failures in #27, and they seem to happen only when unit tests run under a recent Pester build (5.1.0 rather than version 4.10.1 which is the one the test script installs if it finds no Pester).

The test code does not follow the rules for Pester 5.x code (https://github.com/pester/Pester#pester-500).

We may want to investigate porting unit tests to Pester 5.x, or figuring out if there is a way to always run tests with Pester 4.x.

The ubuntu-latest runners for GitHub Actions have Pester 5 installed.