In VS2012, Test Explorer has a bug in that the window doesn’t get focus when you click Test > Windows > Test Explorer. Instead, the menu item has no effect unless the window was complete hidden, in which case it is displayed.
The bug means that if Test Explorer is collapsed with Auto Hide, the menu item does nothing. Check that Test Explorer isn’t in an auto-hide tab somewhere off to the left or right (i.e. in a tab with text written vertically, like Server Explorer and Toolbox are by default).
Update: The bug is fixed in VS2013.