Add tests for `case` statements and other `===` usage

[mschwager]

https://thoughtbot.com/blog/case-equality-operator-in-ruby

I'm not sure if have to use __sanitizer_cov_trace_switch. I think we may be already covering this case since case uses === under the hood, and we're already hooking that. Although it appears we're only doing that for integers. Some investigation is necessary.