test_runner: handled change for exposing 'spec' reporter

Fixes: Issue#48112

Other reporters (dot, tap) by signature are a function while 'spec' reporter is a ES6 class.

This behaviour of api spec is causing difference in semantics while consumption since it has not been addressed anywhere in the document (it has to be instantiated).

Instead of making changes in the signature of spec.js, i have proposed changes where the 'spec' reporter gets exposed in reporter.js

Refs: reporter/spec.js Refs: (@line-no:143) test_runner/utils.js