Adapt test assembly paths in buildscript

appveyor.yml

@@ -14,5 +14,5 @@ build:
   verbosity: minimal
 
 test_script:
-- cmd: dotnet vstest "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.Core.Tests\bin\Release\netcoreapp2.1\MQTTnet.Tests.dll"
-- cmd: dotnet vstest "%APPVEYOR_BUILD_FOLDER%\Tests\MQTTnet.AspNetCore.Tests\bin\Release\netcoreapp2.1\MQTTnet.AspNetCore.Tests.dll"
+- 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"