Add xunit runner to appveyor

[petrsvihlik]

waiting for: http://help.appveyor.com/discussions/problems/1235-uploading-fixie-testresults-as-nunit-or-xunit-gives-500-error

blocker: appveyor/ci#610

tests are currently turned off

docu:
http://www.appveyor.com/docs/appveyor-yml
http://www.appveyor.com/docs/build-worker-api#add-tests
http://www.appveyor.com/docs/running-tests#xunit-c
https://docs.asp.net/en/latest/testing/integration-testing.html <- testserver?

xunit docu:
https://xunit.github.io/docs/getting-started-dnx.html

other resources:
http://blogs.msdn.com/b/webdev/archive/2015/08/06/unit-testing-with-dnx-asp-net-5-projects.aspx
https://fluentbytes.com/running-xunit-test-with-asp-net-dnx-and-tfs-2015-build/

[petrsvihlik]

implemented https://ci.appveyor.com/project/petrsvihlik/wopihost/build/tests

for some reason this was not even needed:

    $wc = New-Object 'System.Net.WebClient'

    $wc.UploadFile("https://ci.appveyor.com/api/testresults/xunit/$($env:APPVEYOR_JOB_ID)", (Resolve-Path .\xunit-results.xml))