perf test: Support dynamic test suites with setup callback and private data

perf test: Support dynamic test suites with setup callback and private data

Add void *priv to struct test_case to allow passing per-test context.
Add int (*setup)(struct test_suite *) to struct test_suite to allow
dynamic generation of test cases. Update build_suites() to invoke the
setup callback for each suite if present, ensuring dynamic cases are
available before listing or running.

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@google.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>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>