Fix doxygen warnings in test code.

Fix doxygen warnings in test code.

- Add missing documentation in partially documented classes.
- Add documentation for gmx::internal and gmx::test::internal
  namespaces (the former adds some warnings outside the tests).
- Fix incorrect tag in paralleloptions.h (unrelated, fixes documentation
  of the gmx namespace).

Change-Id: I14e94235307b3b6e6606bba0b8502b780043cab6