Collect Safari stable results from Buildbot setup

[foolip]

#112 has been resolved and we have reliable results from Safari Technology Preview.

The results for Safari stable are not as reliable, because they run over Sauce and have more timeouts.

Running on the same setup as Safari Technology Preview should resolve this.

[RByers]

This would be great, thanks!

[jugglinmike]

I started looking in to this today. As feared, it wasn't a simple matter of switching the name of the executable. Still, I'm optimistic that it won't take too much more effort.

[foolip]

We have the first Safari 12 results! Interesting comparisons:

• Safari 11 via Sauce vs. Safari 12 local
• Safari 12 vs. Safari TP (both local)

[foolip]

I got so excited that I did a short writeup for webkit-dev:
https://lists.webkit.org/pipermail/webkit-dev/2018-October/030197.html

[jugglinmike]

We resolved this via gh-613 back in October. Closing!