App Insights Alert Guid Parameter

[201-dynamic-web-tests](https://github.com/Azure/azure-quickstart-templates/tree/e71f15c9bfac073235e772db93dc38c75c9b31be/201-dynamic-web-tests)
### Issue Details

Would it be possible to get clarification on the `Guid` parameter in the `azuredeploy.json` template `tests` array? The `README.md` lists it as being "arbitrary". The `azuredeploy.parameters.json` file reuses some guids and creates unique ones in other places. How "arbitrary" is this guid? Can I hardcode one and use the same guid across every resource and resource group? Does reusing a guid cause issues down the road?

If it's a completely vestigial field then we might call that out in some way. @philon-msft or @marcvaneijk might be able to comment as some of the original contributors.

Start with the 201-dynamic-web-tests README.md, azuredeploy.json, and azuredeploy.parameters.json, focusing on the tests array and its Guid values. Determine what the parameter controls and whether reuse is safe; the work is done when the README clearly documents the Guid's purpose, reuse rules, or that it is vestigial.

Written by the indexing model from the issue text.