AzureDevOpsDsc: Fixing non-terminating, integration test errors within `AzureDevOpsProject` resource

[SphenicPaul]

Pull Request (PR) description

Updated the integration tests to remove non-terminating errors in AzDevOpsProject, resource tests when Test-DscConfiguration is called:

• Added -ErrorAction Stop to the Test-DscConfiguration function calls (within the tests) and updated exceptions thrown to make use of New-InvalidOperationException.
• Wrapped possibly exceptions to ensure Test() function within AzDevOpsDscResourceBase class so it always returns a $true or $false (and doesn't throw any exception).

Additionally, updated 'CONTRIBUTING.md' to add additional environment variable to PowerShell script to enable local execution of integration tests and added link to creating PAT within Azure DevOps.

This Pull Request (PR) fixes the following issues

• Fixes #18

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 updated in the resource's README.md.
• [ ] Resource parameter descriptions updated in schema.mof.
• [ ] Comment-based help updated, including parameter descriptions.
• [ ] Localization strings updated.
• [ ] Examples updated.
• [ ] Unit tests updated. See DSC Community Testing Guidelines.
• [x] Integration tests updated (where possible). See DSC Community Testing Guidelines.
• [ ] Code changes adheres to DSC Community Style Guidelines.

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This change is 

[johlju]

Ignore the comment above. I reviewed an older revision.

[johlju]

@SphenicPaul Awesome work on this. I will push the changes to the temp branch, and see if we can enable the integration tests.