Run our reftests with wptrunner

[sgmenda]

#5618, It allows us to run reftests using wptrunner.

I've done the needed changes, as proposed by @jdm .

Hopefully this is it.

Thanks @jdm @SimonSapin @wafflespeanut

[highfive]

Thanks for the pull request, and welcome! The Servo team is excited to review your changes, and you should hear from @Ms2ger (or someone else) soon.

[jdm]

@s142857 There's an issue that's making it difficult to evaluate these changes: this git branch seems to include several commits that are different than servo/master in some way. I recommend resetting the branch to a revision of the official master, then cherry-picking your two commits (git cherry-pick 5fb40ad; git cherry-pick 3a922af) on top of it.

Additionally, it will be much easier to read if the two commits are squashed together in one.

[bors-servo]

☔ The latest upstream changes (presumably #6895) made this pull request unmergeable. Please resolve the merge conflicts.

[sgmenda]

ill do it