Fix and Re-enable Worker.ApplicationInsights.Tests

[satvu]

While fixing typos to reenable tests in #2773, it was discovered that this test suite was failing to build and run. Creating a separate issue to investigate these tests and reenable them. Once reenabled, they need to be added to eng/ci/templates/jobs/run-unit-tests.yml:

- task: DotNetCoreCLI@2
    displayName: Application Insights Tests
    inputs:
      command: test
      arguments: -v n
      projects: |
        **\Worker.ApplicationInsights.Tests.csproj