Add benchmark results to JUnit XML

The "-xml" output of QTest contains benchmark results as expected. However, the "-junitxml" output is lacking them.

This prevents easy integration of the benchmark results into popular CI/Build-Server software such as Jenkins or Atlassian Bamboo.

The actual issue is that there is no specific entry for that in the specification of JUnit XML (https://llg.cubic.org/docs/junit), so a work around needs to be found.

See QTBUG-87318 for a test project and more comments. It only fixed the missing durations, but not the missing benchmark results.