Add logging tests for external DF SDK

As a follow up to https://github.com/Azure/azure-functions-powershell-worker/pull/908, we should add unit tests that ensure we're logging the correct warnings when users fail to properly configure the external DF SDK.

Request comes from: https://github.com/Azure/azure-functions-powershell-worker/pull/908#issuecomment-1377774294