Make unittest independent from the location of the executable.

Summary:
This fixes the check in the unittest when doing
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
which I intend to do globally at some point.

Turns out it's comparing with its own executable location, so we
can use QCoreApplication to get it generically.

Test Plan: Verified to work with ctest and with "./testWaylandServerDisplay"

Reviewers: graesslin, mart

Reviewed By: graesslin, mart

Subscribers: plasma-devel, #frameworks

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D6974