Improve PrettyTable testing

[PrzemekWirkus]

https://linaro.atlassian.net/browse/WPERF-563

This is follow up on https://linaro.atlassian.net/browse/WPERF-562 where I’ve added basic PrettyTable tests after we’ve improved PT here: https://gitlab.com/Linaro/WindowsPerf/windowsperf/-/merge_requests/418.

Definition of done

Check if we can add new unit tests in wperf-test for PrettyTable class, see example usage here: https://gitlab.com/Linaro/WindowsPerf/windowsperf/-/merge_requests/418#note_1651563832

Implement new unit tests if applicable.

See: https://gitlab.com/Linaro/WindowsPerf/windowsperf/-/merge_requests/419