do not install testengine

Summary:
by default users have to opt out of BUILD_TESTING meaning everyone would
by default build and get testengine installed even though it serves no
purpose in production. this also includes distros as I've noticed :O

do not install the engine, same as testplugin isn't getting installed.

(I am actually thinking throwing it away as a whole may make sense; it
serves no real purpose over any other engine)

Reviewers: mart, apol

Reviewed By: apol

Subscribers: broulik, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28463