Add support for an 'overrides' field in validatetest meta structures that
allows changing issue severity levels during test execution. This enables
tests to pass when encountering known issues by downgrading their severity.
The overrides field accepts an array of change-severity structures with:
- issue-id: The issue ID to override
- new-severity: New severity level (critical, warning, issue, ignore)
Currently only change-severity overrides are supported. The feature follows
the same pattern as expected-issues and is only available in .validatetest
files, not .scenario files.
Includes comprehensive documentation and a test case demonstrating the
functionality.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9013>
