D8979: ecm_add_test: Use proper path sep on Windows

[Kevin Funk]

kfunk added a reviewer: cgiboudeaux.

REPOSITORY
  R240 Extra CMake Modules

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

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