test: minor fixes to test-module-loading.js

Use block scope for local variables only used in a small code block.
Fixed inverse arguments in some usages of Assert.strictEqual.

PR-URL: https://github.com/nodejs/node/pull/12728


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>