perf test: Add direct off-cpu tests

Since we added --off-cpu-thresh, add tests for when a sample's off-cpu
time is above the threshold, and when it's below the threshold.

Note that the basic test performed in test_offcpu_basic() collects a
direct sample now, since sleep 1 has duration of 1000ms, higher than the
default value of --off-cpu-thresh of 500ms, resulting in a direct
sample.

An example:

  $ sudo perf test offcpu
  124: perf record offcpu profiling tests                              : Ok
  $

Committer testing:

  root@number:~# perf test offcpu
  126: perf record offcpu profiling tests                              : Ok
  root@number:~# perf test -v offcpu
  126: perf record offcpu profiling tests                              : Ok
  root@number:~# perf test -vv offcpu
  126: perf record offcpu profiling tests:
  --- start ---
  test child forked, pid 1410791
  Checking off-cpu privilege
  Basic off-cpu test
  Basic off-cpu test [Success]
  Child task off-cpu test
  Child task off-cpu test [Success]
  Threshold test (above threshold)
  Threshold test (above threshold) [Success]
  Threshold test (below threshold)
  Threshold test (below threshold) [Success]
  ---- end(0) ----
  126: perf record offcpu profiling tests                              : Ok
  root@number:~#

Suggested-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Howard Chu <howardchu95@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Gautam Menghani <gautam@linux.ibm.com>
Tested-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250501022809.449767-11-howardchu95@gmail.com


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