CTest reports no tests found without -C on multi-config generators

On all platforms, when using the Makefile or Ninja generators, running ctest -L travis -j 16 allows us to run all the tests we normally run on CI. But when using the Visual Studio generator, the same build and test command lines result in "No tests found!!". Discovering that multi-config generators need an extra flag to function properly cost my team about two hours yesterday.

I'm sure we're not the only ones affected by this. The warning would be improved by prompting the user to specify a configuration if CTest can detect that some configuration would result in tests being run.