test: improve the code in test-fs-access

Checklist

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

Affected core subsystem(s)

test

Description of change

• use const and let instead of var
• use common.mustCall to control functions execution
• use assert.ifError instead of assert.strictEqual for errors
• use arrow functions