Fix eina file thread test on Windows

Fix eina file thread test on Windows

On windows, we try to open the "cmd.exe" file, but without the full path
the test fails unless it runs from the system directory.

We now use the full path to test the eina_file_open function.

Reviewed-by: Stefan Schmidt <stefan@datenfreihafen.org>
Reviewed-by: Vincent Torri <vincent.torri@gmail.com>
Reviewed-by: João Paulo Taylor Ienczak Zanette <joao.tiz@expertisesolutions.com.br>
Differential Revision: https://phab.enlightenment.org/D12021