test: cleanup IIFE tests

A number of test files use IIFEs to separate distinct tests from
each other in the same file. The project has been moving toward
using block scopes and let/const in favor of IIFEs. This commit
moves IIFE tests to block scopes. Some additional cleanup such
as use of strictEqual() and common.mustCall() is also included.

PR-URL: https://github.com/nodejs/node/pull/7694


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>