Visual Studio test explorer cannot filter skipped tests by message

[tuespetre]

Since the skip reason is added as a 'standard output' message, it doesn't show the message in the test explorer or allow you to filter by message. I propose to change it so that on a skipped test, the VS test result's 'error message' is set to the skip reason instead, and that the skip reason is not passed as 'standard output'. I have tried this with a clone of the repository and the result is as expected. Please let me know if I can submit a PR.

[clairernovotny]

Please file a PR for review!