The problem is that you are trying to include the TestAssembly definitions in the player build, which is a deprecated behavior. If you want to build for UWP you just need to remember to disable playmode tests for all assemblies from the dropdown menu by right clicking on Test Runner tab.
The `playModeTestRunnerEnabled` flag, which is enabled or disabled from clicking the above-mentioned button, was used before the introduction of assembly definition files.
It is no longer necessary, creating an empty Unity project the flag will by default set to 0 (disabled). If you really need to have the playmode tests for all assemblies enabled to run your tests inside the editor just remember to disable it before building the player.

We recommend putting tests into separate assemblies and using assembly definition files. This way you will have more control over which assemblies need to reference test related dlls.