[Better Quality] Add code coverage

Problem/Motivation

For the sake of better module quality, add code coverages for the tests you have written. Make sure that you do this when you have completed all the 8.x issues.

Proposed resolution

• Find out which service to use. Coveralls, php-code-coverage, and many more
• Make code changes as per the service
• Add code coverage badge to the GitHub repo

Remaining tasks

N/A

User interface changes

N/A

API changes

N/A

Data model changes

N/A