Add automated tests for requests per second graph in HTML report

[slashrsm]

This is a follow-up of #381 and is part of #246.

[slashrsm]

In order to reliably test this hunk I would need to control the return value of Utc::now(). There is a pull request that allows that, but it is still in progress.

Until that is in we could add something on GooseAttack that would wrap Utc::now() and let us override it in tests. Not ideal since we would be polluting code with test-only logic. @jeremyandrews @LionsAd Any ideas/thoughts?

[jeremyandrews]

@slashrsm There's plenty of places in the code not currently tested. Perhaps we add a comment w/ a @todo linking to the PR in question as a reminder to add a test when it's available?