ctest: export test labels to junit xml

With this patch, the junit files written by ctest also contain the
test labels as testcase properties, see
https://github.com/testmoapp/junitxml#properties

The name for the property is "cmake_labels", where "cmake_"
is used namespace-like. This could be done the same way
if we export more properties to junit.
Also this makes it clear to consumers that this is coming
from cmake, and the list is a cmake-formatted string.