Refactor coverage report service

Ref: #267600 (closed)

What does this MR do?

This MR refactors our coverage report service.

It renames our existing CreateArtifactWorker to CoverageReportWorker.

Why are we doing this?

The grouptesting is currently implementing codequality on mr diff.

In order to prepare the work for this feature, we are refactoring our existing worker.

We will add a new CodequalityWorker in a follow up, as explored on this POC.

Screenshots

Database

Migration output

$ bin/rails db:migrate
== 20201224144948 MigrateCoverageReportWorker: migrating ======================
== 20201224144948 MigrateCoverageReportWorker: migrated (0.0008s) =============

Migration Rollback

$ bin/rails db:rollback
== 20201224144948 MigrateCoverageReportWorker: reverting ======================
== 20201224144948 MigrateCoverageReportWorker: reverted (0.0005s) =============

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team