AccountPolicy: Fix applying Account_lockout_duration to Zero

[X-Guardian]

Pull Request (PR) description

This PR fixes being able to set the Account_lockout_duration property of the AccountPolicy resource to zero.

The AccountPolicy unit tests have also been improved and updated to Pester v4 format.

This Pull Request (PR) fixes the following issues

• Fixes #140

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (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.
• [x] Unit tests added/updated. See DSC Community Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [x] New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[codecov[bot]]

Codecov Report

Merging #148 into master will increase coverage by 0%. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #148   +/-   ##
=====================================
  Coverage      89%    89%           
=====================================
  Files           5      5           
  Lines         573    574    +1     
=====================================
+ Hits          514    516    +2     
+ Misses         59     58    -1