perf test: Split monolithic 'util' test suite into sub-tests

perf test: Split monolithic 'util' test suite into sub-tests

Refactor the monolithic 'util' test suite into distinct 'String
replacement' and 'BLAKE2s hash' sub-tests using the struct test_case
framework. This improves test reporting granularity and is used in a
subsequent perf test for JUnit XML test result reporting.

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>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>