Store HTML content in file during the XML parsing

The HTML contents was saved in memory and then stored in files once the whole result XML file has been parsed.
To avoid OOM, I write the files during the XML parsing.

I think we could close JENKINS-15863 & JENKINS-13936.

Note : during my tests, I've detected that the "Detail" link is not working in case of test which is not a suite (I think taht it is linked to pull request 10).