[CI] Display command on failure page

[delanni]

Summary

This PR adds the executed command line to the failures page. We tweak the reporters to export the executed command to the junit xmls, then we read those attributes after parsing the results.

The tests needed some adjustment, because they're very brittle, and don't seem to be very accurate anymore.

Closes: https://github.com/elastic/kibana-operations/issues/127

Check out the [logs] for the failed tests here (ftr/jest/jest_integration): https://buildkite.com/elastic/kibana-pull-request/builds/218457