Auto merge of #38695 - alexcrichton:debug-appveyor, r=brson

Auto merge of #38695 - alexcrichton:debug-appveyor, r=brson

appveyor: Attempt to debug flaky test runs

This commit is an attempt to debug #38620 since we're unable to reproduce it
locally. It follows the [advice] of those with AppVeyor to use the `handle.exe`
tool to try to debug what processes have a handle to the file open.

This won't be guaranteed to actually help us, but hopefully it'll diagnose
something at some point?

[advice]: http://help.appveyor.com/discussions/questions/2898