Adding XCTest unit tests to existing app doesn't work

Adding XCTest unit tests to existing app doesn't work

I've added tests (XCTests) to an existing C++ command line app in Xcode 5
via Test Navigator > (+),
changed the extension of the test class to .mm,
added the XCTest framework to the project.
All compiles fine. Now running the tests just gives me a 'Test failed'
message, nothing in the console and neither green nor red lights in the
Navigator (i.e. no tests executed).
Starting with a fresh Xcode 5 project and changing the extension of the
test class to .mm just works fine so I'd assume it's not just about
lacking support for Objective-C++ in XCTest.
Even with a plain, vanilla test target added to the existing C++ project
the tests fail before ever running.
Any more gotchas to watch out for when adding XCTests to existing
(Objective-)C++ targets?