fix: Clarify PR comment when only pre-existing test failures exist

Fixes #535

The PR comment template only checked extra_failed_tests (new failures
introduced by the PR) when deciding what summary message to display.
When there were common_failed_tests (tests failing on both master and
the PR) but no extra_failed_tests, it would confusingly show a list
of failing tests followed by "All tests passing".

Now uses three-way logic:
- extra_failed_tests present: "not all tests passed" (PR has regressions)
- common_failed_tests only: "no new failures, but pre-existing failures"
- neither: "all tests passed"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: 2 people

templates/ci/pr_comment.txt

@@ -53,9 +53,11 @@ Congratulations: Merging this PR would fix the following tests:
 </details>
 
 {% if comment_info.extra_failed_tests | length %}
-It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you). 
+It seems that not all tests were passed completely. This is an indication that the output of some files is not as expected (but might be according to you).
+{% elif comment_info.common_failed_tests | length %}
+This PR does not introduce any new test failures. However, some tests are failing on both master and this PR (see above).
 {% else %}
-All tests passing on the master branch were passed completely.
+All tests passed completely.
 {% endif %}
 
 Check the <a href="{{ url_for('test.by_id', test_id=test_id, _external=True) }}">result</a> page for more info.