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

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D17130