perf test: Add libsubcmd help tests

perf test: Add libsubcmd help tests

Add a set of tests for subcmd routines.  Currently it fails the last one
since there's a bug.  It'll be fixed by the next commit.

  $ perf test subcmd
   69: libsubcmd help tests                                            :
   69.1: Load subcmd names                                             : Ok
   69.2: Uniquify subcmd names                                         : Ok
   69.3: Exclude duplicate subcmd names                                : FAILED!

Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250701201027.1171561-2-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>