Restore browser tests for Safari

I removed the Safari ones in https://github.com/basecamp/trix/pull/1227. They were using older Safari versions that Solanolabs stopped supporting. I didn't manage to make the newer versions work because Solano had changed the required configuration.

> Desired capabilities are using JWP style which is not compatible with Appium version starting from 2.0. To use Appium version 2.0.0, please use W3C capabilities style since only W3C supports Appium version higher than 2.0Please check our platform configurator for more details