Unhandled Exception running tests under opencover

[SteveL-MSFT]

Steps to reproduce

Start-PSBuild -Configuration codecoverage -Clean -Restore -ResGen -TypeGen -Runtime win10-x64
Ipmo test\tools\opencover\opencover.psd1
Install-OpenCover 
invoke-opencover

Might be related to #2640

Expected behavior

Successfully complete execution of tests

Actual behavior

Unhandled Exception: System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.Collections.ArrayList.ArrayListEnumeratorSimple.MoveNext()
   at Microsoft.PowerShell.PendingProgress.RenderHelper(ArrayList strings, ArrayList nodes, Int32 indentation, Int32 maxWidth, PSHostRawUserInterface rawUI)
   at Microsoft.PowerShell.PendingProgress.Render(Int32 maxWidth, Int32 maxHeight, PSHostRawUserInterface rawUI)
   at Microsoft.PowerShell.ProgressPane.Show(PendingProgress pendingProgress)
   at Microsoft.PowerShell.ConsoleHostUserInterface.ProgressPaneUpdateTimerElapsed(Object sender)
   at System.Threading.TimerQueueTimer.CallCallbackInContext(Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.TimerQueueTimer.CallCallback()
   at System.Threading.TimerQueueTimer.Fire()
   at System.Threading.TimerQueue.FireNextTimers()
   at System.Threading.TimerQueue.AppDomainTimerCallback()

Environment data

Name                           Value
----                           -----
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
PSVersion                      6.0.0-alpha
WSManStackVersion              3.0
PSEdition                      Core
GitCommitId                    v6.0.0-alpha.12
BuildVersion                   3.0.0.0
CLRVersion
SerializationVersion           1.1.0.1