Use ${X11_XTest_LIB} rather than Xtst.

Summary:
Use the cmake variable rather than the library name to link.
This fixes linking on FreeBSD, where the library location is not
in the default linker path.

Reviewers: FreeBSD, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5997