Add a comment to .test_durations explaining where the file came from

[underyx]

Description

Heya! I had this feature request, and figured it's easiest if I create a PR rather than an issue. Let me know if you think it's a good idea; if so I'm happy to put more effort into the PR!

Checklist

• [x] Tests covering the new functionality have been added
• [ ] Documentation has been updated OR the changes are too minor to be documented
• [] The Changes are listed in the CHANGELOG.md OR the changes are insignificant

[jerry-git]

I like the idea of having a "comment" but I think it should not live in the same level as the data for the tests. I guess the initial choice regarding data structure was poor considering extensibility. E.g. something like

{
"test_duration_data": {"test_1": 0.1234, ...}
}

would have been more flexible.