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 browser (Firefox).
Worker is responsible for executing single piece of work and returning it to the Runner. See following diagram that shows sequence of suite processing:
