test: refactor large event emitter tests

This commit splits several large event emitter tests into smaller
block scoped tests. This keeps the tests more isolated. This
commit also replaces var with const, assert.equal() with
assert.stictEqual(), and other small enhancements.

PR-URL: https://github.com/nodejs/node/pull/6446


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>