Update CommonResourceHelper and ResourceHelper tests to Pester 4 standards

[PlagueHO]

Pull Request (PR) description

This PR:

• Update CommonResourceHelper unit tests to meet Pester 4.0.0
  standards (issue #129).
• Update ResourceHelper unit tests to meet Pester 4.0.0
  standards (issue #129).

This Pull Request (PR) fixes the following issues

• Does not fix any issues but contributes to Update Tests to meet 4.x Standards #129

Task list

• Added an entry under the Unreleased section of the change log in the README.md.
  Entry should say what was changed, and how that affects users (if applicable).
• Resource documentation added/updated in README.md.
• Resource parameter descriptions added/updated in README.md, schema.mof
  and comment-based help.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Resource Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Resource Testing Guidelines.
• New/changed code adheres to DSC Resource Style Guidelines and Best Practices.

@johlju would you mind reviewing when you get a moment. Should be straight forward.

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This change is 

[johlju]

This is really strange. The Reviewable button points to the xPSDesiredStateConfiguraiton equivalent. 🤔

[johlju]

LGTM

Comment in GitHub review since Reviewable button points wrongly.

[codecov-io]

Codecov Report

Merging #132 into dev will not change coverage.
The diff coverage is n/a.

@@         Coverage Diff         @@
##            dev   #132   +/-   ##
===================================
  Coverage    83%    83%           
===================================
  Files        19     19           
  Lines      2760   2760           
  Branches      4      4           
===================================
  Hits       2305   2305           
  Misses      451    451           
  Partials      4      4

[PlagueHO]

Oh wow - that is pretty weird about the reviewable. I wonder if this is a long standing issue?

[johlju]

@PlagueHO First time I have seen it. Didn't see the Reviewable status check either, so probably a temporary glitch

[PlagueHO]

Will keep an eye on it.