With Sprint 102 deployment on Visual Studio Team Services, we have enabled tests from within an assembly to be distributed to remote machines using the Run Functional Tests task (formerly called Run Tests using Test Agent). This has been one of the bigger asks from customers around Remote testing. You will see a new check box in the task as below.
Instead of distributing tests at the assembly level, enabling this setting will distribute tests based on the number of machines irrespective of the container assemblies given to the task. Please try it out (see https://blogs.msdn.microsoft.com/visualstudioalm/2015/05/29/testing-in-continuous-integration-and-continuous-deployment-workflows/ for information on how to use the task) and let me know if you have any feedback.