ADGroup: Refactor Module, Unit and Integration Tests (#631)

This PR refactors the module, unit and integration tests of the ADGroup resource as follows:

- Remove non mandatory parameters from Get-TargetResource.
- Remove nested Try/Catch blocks from Get-TargetResource.
- Refactor Test-TargetResource to use Compare-ResourcePropertyState common function.
- Refactor Set-TargetResource to use Compare-ResourcePropertyState common function.
- Modify membership add/Remove to use Compare-Object to only add/remove changed members.
- Fix Set-ADCommonGroupMember function so parameters are not changed.
- Fix Credential parameter description.
- Enhance integration tests to use a data driven pattern.
- Enhance unit tests to use contexts and newer standard test patterns.

CHANGELOG.md

@@ -7,6 +7,12 @@ For older change log history see the [historic changelog](HISTORIC_CHANGELOG.md)
 
 ## [Unreleased]
 
+### Changed
+
+- ADGroup
+  - Refactored Module.
+  - Refactored Unit and Integration Tests.
+
 ### Added
 
 - ADGroup