perf test: Add -j/--junit option for JUnit XML test reports

perf test: Add -j/--junit option for JUnit XML test reports

Add a -j/--junit command line option to generate standard JUnit XML
format test reports. The generated file defaults to 'test.xml' if no
filename is specified, but allows users to override the path (e.g.
-jmytest.xml).

The XML report captures individual test suite and subtest execution
latency, alongside XML-escaped failure logs and skip reasons, while
preserving the full multi-process concurrency speed of parallel test
execution.

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20260602174129.3192312-15-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>