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
• order modules per test writing guide

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test fs