AADServicePrincipal - fix AppId assignment

[niwamo]

Pull Request (PR) description

Fixes the assignment of the AppId for the AADServicePrincipal resource. It's currently being set to $appInstance.DisplayName, which does not get instantiated at all in most cases. Even in those cases where it does, DisplayName is the wrong property. The AADServicePrincipal AppId property exists on the AADServicePrincipal object, which is always defined.

This Pull Request (PR) fixes the following issues

• Fixes AADServicePrincipal: AppId is missing from configuration files and cannot be compiled #5549

Task list

• 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 parameter descriptions added/updated in the schema.mof. (NOT APPLICABLE)
• Resource documentation added/updated in README.md. (NOT APPLICABLE)
• Resource settings.json file contains all required permissions. (NOT APPLICABLE)
• Examples appropriately added/updated. (NOT APPLICABLE)
• Unit tests added/updated. (NOT APPLICABLE)
• New/changed code adheres to DSC Community Style Guidelines.

[niwamo]

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[niwamo]

My mistake for not checking the unit tests the first time. The failing test has been patched.

It seems clear that the AADServicePrincipal's AppId property should be equal to the AADServicePrincipal's AppId (rather than its DisplayName). Unsure why the test was configured this way.