I already covered that Apple has announced announced several testing tools at WWDC 2015 but responsive design mode didn’t get any mention in it.
As a mobile tester, you are constantly spending your time with phones. And not typically on one phone, it is a mix of iPhones, Nexus, Samsung etc. For me typically this runs about 20 hours on these tiny keyboards and screens per day.
The following are the popular ways to test/develop your mobile site.
I have been a very big proponent of slack for team communications. I used hipchat in my last company and pounced on slack when another team in new company is using it.