Update build script.

appveyor.yml

@@ -1,5 +1,5 @@
 version: 1.0.{build}
-image: Visual Studio 2017
+image: Visual Studio 2019
 configuration: Release
 before_build:
 - cmd: >-
@@ -14,5 +14,5 @@ build:
   verbosity: minimal
 
 test_script:
-- cmd: dotnet vstest "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.Core.Tests\bin\Release\netcoreapp2.2\MQTTnet.Tests.dll"
-- cmd: dotnet vstest "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp2.2\MQTTnet.AspNetCore.Tests.dll"
+- cmd: dotnet vstest "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.Core.Tests\bin\Release\netcoreapp3.1\MQTTnet.Tests.dll"
+- cmd: dotnet vstest "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp3.1\MQTTnet.AspNetCore.Tests.dll"