Fix qmakeproject unittest to work on Windows.

Summary: Fix qmakeproject unittest to work on Windows

Test Plan: Build on Windows, verify test passes. Build on Linux, verify test passes (as before).

Reviewers: #kdevelop, kfunk

Reviewed By: #kdevelop, kfunk

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D17158