Worker
The Worker is a single processing unit that can perform a specific task e.g. collect a screenshot using the Firefox browser,
collect a page source, compare two screenshots, check if the source of a page is W3C-compliant and many others.
Worker uses jobs to perform the tests. This module communicates with the browser (Firefox).
The Worker is responsible for executing a single piece of work and returning it to the Runner. See the following diagram that shows the sequence of suite processing:
