Potential fix for flaky compliance dashboard spec

What does this MR do and why?

This spec might be flaky because the first merge request defaults with a nil merged_at value, but the test runs in this sequence:

visit group_security_compliance_dashboard_path(group)
merge_request.metrics.update!(merged_at: merged_at)
wait_for_requests

If the page is loaded before the metrics are updated, it's possible that a row is missing from the dashboard.

To fix this, visit the page only when the merge request metrics have been updated.

Relates to #381137 (closed)

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