perf offcpu: Update offcpu test for child process

Record off-cpu data with perf bench sched messaging workload and count
the number of offcpu-time events.  Also update the test script not to
run next tests if failed already and revise the error messages.

  $ sudo ./perf test offcpu -v
   88: perf record offcpu profiling tests                              :
  --- start ---
  test child forked, pid 344780
  Checking off-cpu privilege
  Basic off-cpu test
  Basic off-cpu test [Success]
  Child task off-cpu test
  Child task off-cpu test [Success]
  test child finished with 0
  ---- end ----
  perf record offcpu profiling tests: Ok

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Blake Jones <blakejones@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20220811185456.194721-5-namhyung@kernel.org


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