CT: Add support for infinite loop testing

[zepumph]

From discussion with @jonathanolson over in phetsims/axon#447, we thought there could be better support for CT to know when an infinite loop is. The solution in my mind is to add a new event that is added to each browser test: "test start". That way we don't overload what loading the page means, and we have better data about if we just never started the test (took too long to load (totally normal and fine)), or started but then never got a pass/fail (infinite loop?).

This may not be too hard, but wouldn't be something I could just squeeze in. It may be important enough over in phetsims/axon#447, but I doubt it will raise the priority of this issue.