Where is the 'tests output pane'?

The message really should be this

Unexpected error detected. Check the Output Window for details which can be found in the Show output from: section. Then select the Tests drop down to read the actual error.

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Why the Confusion?

For the Tests window is, buried, in the output window. That window acts as the main window that shows the textual Build information; and hides the test result info in another dropdown.

So in the OutputWindow select Tests after a run.

In the output window there is combobox "show output from". Choose Tests as shown here

Go to the view tab at the top of your screen, and select output.