[autotests/integration] Simplify the cmake for creating a test

Summary:
A dedicated cmake function is added to create a test case. It takes
a NAME, the SRCS and additional LIBS.

Thus it's
integrationTest(NAME myTestCase SRCS test.cpp)

to create a standard integration test.

In addition kwin_wayland_test.cpp is compiled into a static library
to decrease the compile time a little bit.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2052