[tests-only] allow underscore in expected-failures links

[phil-davis]

Description

A lint checker for expected-failures files was added to CI in PR #39782
One of the checks is to find and validate the link that should be in () after an expected-failures feature file mention.
Links can contain an underscore character, e.g.:

-   [apiFilesSpaces/filesSpaces.feature:114](https://github.com/owncloud/files_spaces/blob/master/tests/acceptance/features/apiFilesSpaces/filesSpaces.feature#L114)

The repo is files_spaces.

This PR adds the _ to the allowed characters used to find matches of links.

How Has This Been Tested?

CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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