[gitlab] Fetch macOS unit test reports

[KSerrania]

What does this PR do?

Modifies the tests_macos job to fetch artifacts from the triggered GitHub Action.

Requires https://github.com/DataDog/datadog-agent-macos-build/pull/104 to be merged first.

Motivation

Once https://github.com/DataDog/datadog-agent-macos-build/pull/104 is merged, the macOS tests GitHub Action will start producing JUnit test reports as artifacts, which will be picked up by the Gitlab job, and stored as an artifact which will be sent by the source_test_junit_upload job.

Additional Notes

Example test report in CI App.

Possible Drawbacks / Trade-offs

n/a

Describe how to test/QA your changes

Check that macOS test results are shown in the CI App UI.

Reviewer's Checklist

• [x] If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• [x] Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• [x] A release note has been added or the changelog/no-changelog label has been applied.
• [x] Changed code has automated tests for its functionality.
• [x] Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• [x] At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• [x] If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• [x] If applicable, the need-change/operator and need-change/helm labels have been applied.
• [x] If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• [x] If applicable, the config template has been updated.

[KSerrania]

Sounds good in general! agent-shared-components' review is for release.json which I think will change back to master when this lands?

Yes, that's right, once the linked datadog-agent-macos-build PR is merged (after the freeze).