perf tests: Include error output for skipped tests in JUnit XML

The JUnit XML output correctly captures the stderr/stdout output of failed
tests inside the <failure> element. However, for skipped tests, the output
was completely discarded and the XML only received a self-closing <skipped
message="reason"/> tag.

This expands the <skipped> element to include the test's err_output when
available, which is extremely helpful for debugging why a test was skipped
(e.g. diagnosing missing prerequisites or unexpected environment states
that triggered the skip) directly from CI systems parsing the XML report.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

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