Ensure that only one test run is active at a time

[TylerLeonhardt]

Right now you can multi-select tests and they will trigger X separate runs. That's not ideal. We should only run one since that's all the PowerShell extension can handle at a time.

Perhaps Pester could also support specifying multiple It's or Describe's to run in a file. (cc @nohwnd)

All in all, I think this scenario is small and for now we should just say "it's not supported".

[nohwnd]

That already works, just pass mulitple files to Path and more line filters to line filter. And if it does not work it is a bug.

[TylerLeonhardt]

We'll need a way in vscode-powershell to specify those things.