Support user-based authentication in live tests

[mccoyp]

Description

Adds support for Azure PowerShell and CLI authentication, building on new support for user authentication with live test resources -- see Azure/azure-sdk-tools#7580.

Since most users deploy test resources with PowerShell via New-TestResources.ps1, I gave AZURE_TEST_USE_PWSH_AUTH preference over AZURE_TEST_USE_CLI_AUTH when both are set to "true".

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[lmazuel]

LGTM

[scbedd]

Is importing the AzureCLICredential literally all you need to do here? That's pretty legit.

[mccoyp]

Is importing the AzureCLICredential literally all you need to do here? That's pretty legit.

Yep! As long as users are using the get_credential method on the test class -- which is the recommended flow -- they'll pick up the change. Fortunately the transition to user-based auth seems a lot simpler than we might have expected.