Fix projectmodel unittest to work on Windows.
Summary: Fix projectmodel unittest to work on Windows. Use paths according to the platform we are using
Test Plan: Compile and test on Windows (Debug build) to see the unittests are passing. Compile and test on Linux to see that the test pass (As before)
Reviewers: KDevelop, kfunk
Reviewed By: KDevelop, kfunk
Differential Revision: https://phabricator.kde.org/D17130