🦟 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)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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