Summary:
Adding this definition makes cmake rebuild **everything** when toggling
BUILD_TESTING switch. With the patch applied, one can switch on and
off tests without recompiling anything.

Test Plan:
1) Build Krita with tests (takes 28 min on 4+4 CPU)
2) Disable the tests
3) Type: 'make -j8 install'

Without the patch cmake will do a full recompile without the tests,
which takes 22 min. With the patch, cmake will recompile a couple of
files only, taking about 12 seconds(!).

Reviewers: #krita, rempt

Differential Revision: https://phabricator.kde.org/D9674