[WIP][qa] experimental: run browsertack jobs in parallel.

[felipesanches]

This is a followup to pull request #246 that was automatically closed by mistake when this project renamed its "master" branch to "main".

Be sure to also read all the comments posted in the original PR (#246)

Original PR description as submitted on Aug 26, 2020 by @graphicore:

See googlefonts/diffbrowsers#26 and googlefonts/diffbrowsers#28

---

Be sure to also read all the comments posted in the original PR (#246)

[google-cla[bot]]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

ℹ️ Googlers: Go here for more info.

[m4rc1e]

Closing since we've dropped diff browsers.