Test Studio Run-time edition does not deploy the TestAdapter.dll with its installation.
Thus Test Studio tests cannot be executed with the VSTestConsole.exe in a Azure DevOps (VSTS) pipeline.
The TestAdapter.dll for VS 2017 and VS 2019 is being deployed to the Test Studio installation folder (for Ultimate or Dev Edition installations) along with the respective *.vsix package. Until the misbehavior is fixed, the vsix package can be copied to the agent machine and installed with double click. Then refer the dll from the respective location.