Durations missing in JUnit XML

When using the JUnit XML format as test output format, the benchmark tests and the test's durations are missing, although the JUnit XML format explicitely includes an optional "time" attribute for each "testcase" entry (see https://llg.cubic.org/docs/junit).

For the xml format, they are both included.

This should at least be mentioned here until fixed: https://doc.qt.io/qt-5/qtest-overview.html