Add Go example for the test coverage visualization feature

What does this MR do?

This commit adds a Go/Golang based .gitlab-ci.yml example to demonstrate the coverage visualization feature. It uses go test for test execution and coverage profiling while gocover-cobertura is used for creating Cobertura XML report.

We use this feature in combination with Go since quite a while and it works rather well. However, due to the nature of Go's coverage profiling, getting it to work is not that trivial. That's why an example like this could already help.

This example has been validated within a test project (see fh1ch/hello-inline-coverage-golang!3 (diffs)) and is also pretty much identically in use within the gitlab-org/gitlab-runner project (see https://gitlab.com/gitlab-org/gitlab-runner/-/blob/main/Makefile#L146).

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