The test_runner.py script generates a JUnit XML ouput file which is
used by the CI to offer comprehensive test report.
The default for it is a constant file name located in the build
directory. The result is that subsequent calls to test_runner.py will
override the previous reports if an explicit file name is not passed.
This diff keeps the file name the same but moves it to the temporary
directory, which is unique to the runner. It avoids collisions, makes it
easier to find the report and makes it more obvious which runner it