[df] Make Report action mergeable and available in distributed RDF

[martamaja10]

The report action has not been mergeable so far and hence it was not possible to use it in distributed RDF. This PR makes it possible.

Note:

• distributed RDF tests to be added
• for now, merged report is tested within the test_report instead of merge_results since it requires a ref file - to be decided where is best to keep this test.

[github-actions]

Test Results

    22 files      22 suites   3d 15h 33m 6s ⏱️
 3 775 tests  3 775 ✅ 0 💤 0 ❌
75 079 runs  75 079 ✅ 0 💤 0 ❌

Results for commit 31457de.

♻️ This comment has been updated with latest results.