ctest: skip tests if the label has no tests defined
ctest: skip tests if the label has no tests defined

This avoids error code 255 for cases where a label has 0 tests. This
resulted in spurious Travis & AppVeyor failures.

Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,VcDevel/Vc