Add GitLab Test Coverage Visualization to Karma, Jest and RSpec

What does this MR do?

This MR enables the GitLab Test Coverage Visualization feature for the gitlab-org/gitlab repository. It adds the Cobertura exporter to Jest, Karma (via Istanbul code coverage tooling; Cobertura is available by default) and all rspec based tests (via simplecov-cobertura gem). The reports are then collected via GitLab-CI artifacts:reports:cobertura feature.

Note: The Test Coverage Visualization feature is currently behind a feature-flag (:coverage_report_view). While the reports can already be recorded without side-effects, displaying the coverage will only happen if the feature-flag is enabled or the feature reaches GA.

Screenshots

Example of a gitlab-org/gitlab repository MR on an instance with enabled feature-flag.

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