c++: tests/unittests: add extern C in header files

This PR adds `extern "C"` for the header files in `./RIOT/test/unittests` folder.
`./RIOT/test` does not contain any header

~~I left out `./unittests/embUnit/embUnit.h` as it only collects all other headers form `embUnit`~~ (also encapsulated it to eventually satisfy #1789)
un-related newline ! :P