Fix and Re-enable `Worker.ApplicationInsights.Tests`

While fixing typos to reenable tests in https://github.com/Azure/azure-functions-dotnet-worker/pull/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
```

Start by locating Worker.ApplicationInsights.Tests.csproj and reproducing why the test suite fails to build and run. Check the related test project and the existing CI configuration in eng/ci/templates/jobs/run-unit-tests.yml. Done means the tests build and run successfully, then the Application Insights test task is added to that CI template.

Written by the indexing model from the issue text.