Stronger Verification for Azure Credential Factory test

[Michael-A-McMahon]

The test for TokenCredentialFactory will now invoke getToken() on the TokenCredential. This will light up some additional Azure SDK code, and catch defects like this: Azure/azure-sdk-for-java#39002

The defect linked to above will cause AZURE_DEFAULT/auto-detect authentication to fail when a AZURE_CLIENT_SECRET and AZURE_CLIENT_CERTIFICATE are not configured. Users may hit this when attempting to use AZURE_USERNAME and AZURE_PASSWORD for authentication.

Users can work around this by pulling in the latest azure-identity module in their own pom.xml:

    <dependency>
      <groupId>com.azure</groupId>
      <artifactId>azure-identity</artifactId>
      <version>1.12.1</version>
    </dependency>

[Michael-A-McMahon]

This is a real tricky one to catch! Notice how our CI run did not fail? That's because we have set AZURE_CLIENT_SECRET as an environment variable. When we do that, the DefaultCredential won't attempt to use the AZURE_USERNAME and AZURE_PASSWORD, even though we've set those as environment variables too.

Only way to really verify scenarios like this is to have two separate test runs which use different environment variables. So while the changes in this branch do add some value, they are not doing as much as I had hoped. To truly verify all scenarios, we'll need future work on the CI run. But for now, I'll add this test scenario to the list of those I verify manually before a release (the list already includes interactive authentication methods, might as well add this too) .