Revert "Fix windows build"

That patch was wrong. BUILD_TESTING needs to always be defined when
testing is enabled so the *_TESTS_EXPORT macros are filled out properly.

I think this patch initially just worked around an issue which was fixed
by https://phabricator.kde.org/D20091.

Phabricator Revisino: https://phabricator.kde.org/D28759

This reverts commit 2f552ac573770ce22ec5489b22c6269280aa4e36.

Reviewers: vonreth, dvratil, mlaurent, dfaure, winterz

Reviewed By: mlaurent, dfaure

Subscribers: kfunk, vonreth, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28759