test: cleanup/update test-dgram-empty-packet.js

Checklist

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

Affected core subsystem(s)

Description of change

• Changed some var to const and 'let'
• Changed == to includes for clarity.