Update appveyor.yml

appveyor.yml

@@ -13,4 +13,4 @@ build:
   project: MQTTnet.sln
   verbosity: minimal
 test_script:
-- cmd: dotnet vstest /logger:Appveyor "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.Core.Tests\bin\Release\netcoreapp2.0\MQTTnet.Core.Tests.dll" /UseVsixExtensions:true
+- cmd: dotnet vstest /logger:Appveyor "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.Core.Tests\bin\Release\netcoreapp2.0\MQTTnet.Core.Tests.dll"