Update tls client test

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)

tests

Description of change

Update tls test with

• to use const/let for definition of variables (instead of using var).
• update equal assertions with strict equality check to ensure deeper comparison of expected value with return value
• wrap function callbacks in with common.mustCall to avoid repeated calls