WIP #217 - Use 'nunit' endpoint

This follows the advice at https://help.appveyor.com/discussions/problems/37319-http-500-error-when-uploading-nunit3-test-results-suspected-failure-on-parameterized-test-cases

Apparently using the regular nunit ebony works, where the nunit3 endpoint does not.

Tools/appveyor-upload-test-results.ps1

@@ -13,5 +13,5 @@ foreach($project in $TestProjects)
 {
     $testResultFile = "$SolutionRoot\Tests\$project\TestResults\TestResults.xml"
     Move-Item $testResultFile "$SolutionRoot\TestResults\$project.TestResults.xml"
-    $wc.UploadFile("https://ci.appveyor.com/api/testresults/nunit3/$($env:APPVEYOR_JOB_ID)", (Resolve-Path $SolutionRoot\TestResults\$project.TestResults.xml))
+    $wc.UploadFile("https://ci.appveyor.com/api/testresults/nunit/$($env:APPVEYOR_JOB_ID)", (Resolve-Path $SolutionRoot\TestResults\$project.TestResults.xml))
 }