Add file attribute to JUnit report testcase element

[el7cosmos]

Description

This PR add file attribute in the <testcase> element so that GitLab's JUnit report can show a link to the related file.

Suggested changelog entry

Related issues/external references

Related to #2964

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
  • [ ] This change is only breaking for integrators, not for external standards or end-users.
• [ ] Documentation improvement

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the Contribution Guidelines.
• [x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• [ ] I have added tests to cover my changes.
• [x] I have verified that the code complies with the projects coding standards.
• [ ] [Required for new sniffs] I have added XML documentation for the sniff.
• [ ] [Required for new files] I have added any new files to the package.xml file.