test: refactor the code in test-fs-watch.js

• used let and const instead of var
• used assert.strictEqual instead assert.equal

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

used let and const instead of var used assert.strictEqual instead of assert.equal