test: refactor the code in test/pummel/test-child-process-spawn-loop.js

• 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
• commit message follows commit guidelines

Affected core subsystem(s)

test

change description:

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