I am able to successful runs MSTest unit tests in Visual Studio 2017 but it would not run on the TFS 2018 server (or TFS 2015 before I upgraded).

While setting up TFS 2018 to run unit tests I was getting the following warnings.

Warning: [MSTest][Discovery][C:\Agent\_work\2\s\[......]
Warning: No test is available in C:\Agent\_work\2\s\[......]

Information: Additionally, you can try specifying '/UseVsixExtensions' command if the test discoverer & executor is installed on the machine as vsix extensions and your installation supports vsix extensions.

##[warning]No results found to publish.
##[section]Finishing: VsTest - testAssemblies

THE FIX

By default the "Search folder" was set to $(System.DefaultWorkingDirectory) that path was looking at c:\agent\_work\2\s\MyProject.Tests\obj\Release\MyProject.Tests.dll but that folder did not have the following DLLs.   

Microsoft.VisualStudio.TestPlatform.MSTest.TestAdapter.dll
Microsoft.VisualStudio.TestPlatform.MSTestAdapter.PlatformServices.dll
Microsoft.VisualStudio.TestPlatform.MSTestAdapter.PlatformServices.Interface.dll

Changing the  "Search folder" to $(Build.StagingDirectory) corrected the issue and the test ran successfully.

Here are are the tasks in the build.

Here is where the value for the path had to be updated.