Improve accuracy of colcon-test-result

[scpeters]

🦟 Bug fix

Helps with gazebosim/sdformat#1648

Summary

While experimenting with action-ros-ci in gazebosim/sdformat#1648, I noticed that colcon test-result incorrectly identifies a failed test in a gz-tools build folder even when tests haven't been run due to the .xml suffix of the intermediate ${TEST_NAME}_fail.xml file (also known as UNIT_gz_TEST_fail.xml).

Adding a suffix to the intermediate junit xml files prevents colcon test-result from reading them as real test results.

To test:

1. create a colcon workspace containing gz-tools
2. colcon build
3. colcon test-result should not show a failure
4. colcon test && colcon test-result still should not show the spurious failure

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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[scpeters]

to verify this fix, I cherry-picked on top of gz-tools2 in 1cf2e36 (gz-tools2...scpeters/backport-colcon-test-result) and rebuild sdformat16 from source against this branch:

• failure with gz-tools2: https://github.com/gazebosim/sdformat/actions/runs/23859809660/job/69563086769

build/gz-tools2/src/UNIT_gz_TEST_fail.xml: 1 test, 0 errors, 1 failure, 0 skipped
  - UNIT_gz_TEST GZ_SCRIPT
    <<< failure message
      UNIT_gz_TEST failed.
    >>>

• success with scpeters/backport-colcon-test-result: https://github.com/gazebosim/sdformat/actions/runs/23860580111/job/69565842255?pr=1648