Execute xunit/opencover as the test_script on appveyor

[skip ci]

Author: craigminihan

.appveyor.yml

@@ -10,5 +10,7 @@ before_build:
 
 build_script:
  - cmd: msbuild /p:Configuration=%CONFIGURATION% HttpWebClient.sln
+ 
+test_script:
  - cmd: buildtools\OpenCover.4.6.519\tools\OpenCover.Console.exe -register:user -target:"buildtools\xunit.runner.console.2.2.0\tools\xunit.console.exe" -targetargs:"HttpWebClient.UnitTests\bin\%CONFIGURATION%\HttpWebClient.UnitTests.dll -noappdomain" -filter:"+[HttpWebClient]*"