Tests should not rely on external websites which lead to random failures #2504
Description
Today https://httpbin.org/ is down the whole day.
It caused two daily builds to fail with 43 tests for Invoke-WebRequest and Invoke-RestMethod.
https://ci.appveyor.com/project/PowerShell/powershell-f975h/build/6.0.0-alpha.10-26
https://ci.appveyor.com/project/PowerShell/powershell-f975h/build/6.0.0-alpha.11-27
We should replace httpbin with eaver our own hosted version or localhost server, i.e.
https://github.com/Runscope/httpbin
localhost server will introduce unnecessary testing dependencies, not sure which one would be a better solution.