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.
Workaround:
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.