test: refactor test/sequential/test-fs-watch.js

* add block scoping
* rename block-scoped identifiers (e.g., filenameTwo -> filename)
* use common.mustCall() instead of exit handler
* use common.mustNotCall() as appropriate
* order modules per test writing guide

PR-URL: https://github.com/nodejs/node/pull/14534


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>