text: Title: Elapsed time is ignored when parsing output.xml if start time is not set
  Body: When writing execution times output.xml, we always include the elapsed time and also include the start time if it is set. Typically the start time is available, but it is cleared, for example, when merging results.
  
  The code parsing execution times from output.xml tries to detect is the format new (RF >= 7.0) or old (RF < 7.0) by looking is the `start` attribute set. In the old format there's `starttime` instead of `start`, but `start` is also missing if the suite didn't have the start time set. In such cases the elapsed time is fully ignored. This is easy to fix by looking at the `elapsed` attribute instead of `start`.
  
  If elapsed time is not parsed, it's calculated automatically based on child element elapsed times. That is typically the right time, so it's unlikely that anyone is actually affected by this bug. I noticed it when creating a test for #5058 and it also happens to work as a workaround for that issue.