Fix false errors and broken CI-logs from P-tests

[fflaten]

PR Summary

After switching to -EncodedCommand for child powershell/pwsh processes in P-tests (#2267) we get CliXML-output in CI logs and false errors in CI-report.

Using -EncodedCommand changes default output to XML in non-interactive environments with redirected output like our AzDO Pipelines CI. The normal workaround of explicitly setting -OutputFormat Text is not honored prior to PS 6.2 (incl. 5.1).

This PR reverts to using -Command with different commands to handle the changed argument-behavior in PS 7.3+. It also extracts the helper to a PTestHelpers module to reduce duplicated code.

Fix #2340

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[fflaten]

Workaround solved PS6+ but not 5.1 for some reason. 😕
Worst of all - can't repro locally in PS5.1

[fflaten]

Looks like the logic was changed in PS6+ to respect explicit outputformat (PowerShell/PowerShell#8115), so 5.1 has this logic.

https://github.com/PowerShell/PowerShell/blob/2f818615bed15141c062dd185f659ed110d9c6ba/src/Microsoft.PowerShell.ConsoleHost/host/msh/ConsoleHost.cs#L1261-L1267

AzDO likely redirects errors unlike my local repro attempts

[fflaten]

No solution for PS 5.1 AFAIK, so will revert to -Command.