[CI] Reenable stable build tests

[michaelblyons]

Expected behavior

Do we want to undo the #4018 change and start CI testing against stable builds again? Or are we going to flip-flop on and off enough that it's only worth testing the latest dev build?

Actual behavior

Stable builds currently disabled.

Steps to reproduce

1. Make a PR
2. Push
3. See tests run against dev build only

[jrappen]

We can run stable Builds as optional (i.e. continue-on-error) like before.

If you followed the discussion in the PR introducing running parallel tests, it was designed this way. It was more meant to make us aware that something new is in the dev builds should SublimeHQ slip in some commits into master themselves. And as a heads up for plugin-devs with regard to syntax engine changes. Also there are a lot more tests now, not only for scopes but symbols and transformed symbols.

[deathaxe]

Probably not such a good idea due to syntax tests of #4195 failing as symbol tests rely on fixes of ST4196 or 4197+.