perf test: Fix subtest status alignment for multi-digit indexes

perf test: Fix subtest status alignment for multi-digit indexes

When running perf test, the status column (: Ok) became misaligned when
subtest indexes reached 2 or 3 digits (e.g. 9.100 vs 9.9 vs 10.1). This
occurred because the subtest description field width (subw) was
statically fixed to width - 2, assuming all subtest index prefixes were
exactly 7 characters wide.

Dynamically calculate subw based on the exact character length of the
test suite and subtest index prefix. This ensures the status column is
perfectly aligned vertically across all test outputs regardless of
subtest index digit count.

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>