OK - sometimes it just to confess - I took a chance, or didn’t know what I was doing - call it what you want.
In TFS Build-scripts there are a much nicer way of calling MSTest than the Exec-task as I suggested. There is already a task for it, called TestToolsTask. With this task you can much easier reach all the properties you need. And it even contains some undocumented features such as the TestContainer-property.
Here is my updated version on how to call the task:
<TestToolsTask TestContainers="$(BinariesRoot)\Debug\Company.Service.Host.Test.Integration.dll" BuildFlavor="%(ConfigurationToBuild.FlavorToBuild)" Platform="%(ConfigurationToBuild.PlatformToBuild)" PublishServer="$(TeamFoundationServerUrl)" PublishBuild="$(BuildNumber)" PathToResultsFilesRoot="$(TestResultsRoot)\..\IntegrationTestResults" TeamProject="Betalningsplattformen2" ContinueOnError="true" />