message: Make unit tests a subproject to enable all-build on Windows
diff: # terms of the BSD-3 license. We welcome feedback and contributions, see file
  # CONTRIBUTING.md for details.
  +project(mfem-unit-tests NONE)
  +
  # Include the top mfem source directory - needed by some tests, e.g. to
  # #include "general/text.hpp".
  include_directories(BEFORE ${PROJECT_SOURCE_DIR})
  