test: refactor test-fs-fsync

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

• replace var with const.
• remove successes variable.
• use assert.ifError() for handling all errors.
• wrap all callbacks with common.mustCall().