test: improve the code in test-util-debug.js

[edsadr]

• use const and let instead of var
• use assert.strictEqual instead of assert.equal
• use arrow functions

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

[evanlucas]

LGTM

CI: https://ci.nodejs.org/job/node-test-pull-request/5670/

[italoacasas]

the console.log() is part of the test functionality ?

[edsadr]

@italoacasas that console.log is required, the test is spawning that function and asserting later the output, so if we remove the console log then we won't have anything to assert at the output ... but I removed another one that wasn't required in the same commit

[thefourtheye]

There is already #10531

[edsadr]

@thefourtheye .. right... I am closing this one if favor of #10531 and suggesting there to implement arrow functions to match mine