Remove gocovmerge

What does this MR do?

Replace gocovmerge with native tools for test coverage.

A coverage profile is generated with go test -cover and it's then used to generate a html report of the results.

Closes: #251 (closed)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines