Refactor use of AVOGADRO_DATA_ROOT to find test data

Move find_path(...) call to top level CMakeLists.txt.
Add message if data directory was not found for
directories that need test data.

Change-Id: I663b8bf51d9bd99abb961698c83aa72b9af48e17
Merge topic 'avogadro-data-dir' into master

a128ab66 Refactor use of AVOGADRO_DATA_ROOT to find test data