Support for interactive debugging of tests

It would be neat to generate, say, a $RULE_debug rule for every closure_js_test that would boot a server that allowed one to run a test from any browser. Such a system would make it simpler to debug these tests interactively.

Start by examining the existing closure_js_test rule and how its generated test targets are launched. Investigate what a browser-accessible server would need to run an individual test interactively, then define the expected $RULE_debug target behavior and how completion would be verified.

Written by the indexing model from the issue text.