Run affected tests in Edge and Safari for PRs

[foolip]

One upon a time we had Chrome, Edge, Firefox and Safari all running some tests for PRs. It was never robust enough to be truly useful, but we had it until we had to remove Edge and Safari in #9903.

We are now in the process (#13194) of moving Chrome and Firefox to Taskcluster and we'll use the results in #7475.

That still leaves Edge and Safari without any coverage on PRs.

For Edge, there doesn't appear to exist an CI integrated with GitHub where we could run Edge, much less Edge Insider Preview. @thejohnjansen for ideas.

For Safari, it's possible we could use Travis's macOS support, but nobody has tested this. It would also be a fairly different setup from what @jugglinmike has set up. @youennf for ideas.

The value of this would be preventing accidental regressions of tests in Edge and Safari, just like for Chrome and Firefox.

[foolip]

travis-ci/travis-ci#7187 as some discussion on Safari in Travis.

[foolip]

In foolip#3 I've experimented with macOS on Travis to the point where I got Safari TP installed, do nothing with it, and then run a lint that fails:
https://travis-ci.org/foolip/web-platform-tests/builds/435885997

(For Safari 12 stable, the Travis macOS image wasn't new enough, "This update requires macOS version 10.13.6 or later", but Travis's latest is 10.13.3.)

Put together with some of @jugglinmike's amazing tricks for macOS in https://github.com/web-platform-tests/results-collection it would probably be possible to run affected tests on Safari TP on Travis.

Whether we run them 10 times for stability or just once for results, this will time out if too many tests are affected. We'd also have to find a way to get the results into wpt.fyi. So it's not a trivial undertaking, but at least the OS images exist and probably work for this.

[foolip]

I've also experimented briefly with Cirrus CI. Their macOS support is ultimately backed by https://www.macstadium.com/, which I recall @jugglinmike has mentioned at some point.

Cirrus CI has macOS 10.13.5, and in foolip#4 I got it as far as with Travis, namely I got STP 65 installed.

There's also https://circleci.com/build-environments/xcode/osx/ but I haven't tried it.

Overall, for macOS, it appears that the infrastructure exists that would allow us to get this off the ground.

[foolip]

Also worth noticing is that https://cirrus-ci.org/guide/macOS/ points to https://github.com/cirruslabs/osx-images and that perhaps custom images are possible. It's probably not necessary though, installing STP didn't take long.

[chauser123]

Whatvdo you suggest i do? Should we try removing safari?

[foolip]

@chauser123 no action is required on your part unless you want to work on the Azure Pipelines setup.

[foolip]

Edge: explored in #14300

Safari: #14156

[foolip]

This is done for Safari, but I'm going to close this without doing it for Edge given that it will no longer be actively developed, and so making changes to PRs on the basis of results from Edge doesn't seem like a good use of time. In #15174 I will still try to get full daily runs, however.