inconsistent test count reported in Jenkins if tests are flaky

[dirk-thomas]

The nightly jobs reruns failed test with ctest --rerun-failed. But since this run overwrites the generated Test.xml Jenkins will report a different number of test because tests which passed in the initial run are not included in the new result file anymore. I don't think there is much we can do (except parsing the result file before and after and implementing our own merge strategy). I have reported the problem upstream, maybe it gets addressed in CTest directly: https://gitlab.kitware.com/cmake/cmake/issues/16314