test: update var to let const in tests

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)

Test - test/parallel/test-tis-on-empty-socket

Description of change

Updated the var to const and lets in test-tis-on-empty-socket. This is coming from node interactive - code & learn