Output coverage to Cobertura report

What does this MR do?

• Enables coverage in the unit test job
• Installs github.com/t-yuki/gocover-cobertura and uses it to convert the coverage reports to a Cobertura XML report
• Uploads the XML report as a cobertura artifact.

Notes:

• The first commit (ec1db755) is not supposed to be merged, it is only there to make this MR exemplify how the inline coverage would show up in an MR.
• the GitLab app has issues interpreting the report generated by gocover-cobertura as-is, namely:
  • it cannot interpret more than 1 <source> nodes.
  • it needs to have either absolute or relative paths in filename attributes, instead of prefixed with the package name.

Why was this MR needed?

See #25350 (closed)

What's the best way to test this MR?

• Check for inline coverage indicators once the coverage_report_view feature is enabled on GitLab.com.
• Check that !2252 (merged).json shows a JSON file containing data related to the 2 Go files changed.

What are the relevant issue numbers?

Closes #25350 (closed)