TFS 2015 with x64 Unit Tests

I use TFS 2015 with CI and after the successful build for x64 the unit test should be run in x64, too. But instead they run in x86 which leads to a failure for most of the tests.

To build for x64 it was enough to set the $(BuildPlatform) variable to x64, but for the test, this doesn't have any effect.

I get output for the test like this:

2016-05-03T06:42:38.7749398Z Microsoft (R) Test Execution Command Line Tool Version 14.0.25123.0 2016-05-03T06:42:38.7749398Z Copyright (c) Microsoft Corporation. All rights reserved. 2016-05-03T06:42:38.9155623Z Starting test execution, please wait... 2016-05-03T06:42:39.4155556Z Test run will use DLL(s) built for framework Framework45 and platform X86. Following DLL(s) will not be part of run: ...

What do I have to do, to get TFS to test with x64?

Thx for your help!