D8979: ecm_add_test: Use proper path sep on Windows

[Kevin Funk]

kfunk added a reviewer: bcooksley.

REPOSITORY
  R240 Extra CMake Modules

REVISION DETAIL
  https://phabricator.kde.org/D8979

To: kfunk, dfaure, cgiboudeaux, bcooksley
Cc: #windows, #frameworks, #build_system