[CMake] Add possibility to copy files for ROOT_ADD_GTEST
[CMake] Add possibility to copy files for ROOT_ADD_GTEST

- Contrary to the documentation, ROOT_ADD_GTEST does not react to
`COPY_TO_BUILDDIR`. This adds the missing functionality.
- Further add documentation for existing functionality for
ROOT_ADD_TEST.

karies/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root