perf test: Add truncated perf.data robustness test

Add a shell test that verifies perf report handles truncated perf.data
files gracefully — exiting with an error code rather than crashing with
SIGSEGV or SIGABRT.

The test records a simple workload, then truncates the resulting
perf.data at four offsets that exercise different parsing stages:

  8 bytes   — file header magic only
  64 bytes  — partial file header (attr section incomplete)
  256 bytes — into the first events (partial event headers)
  75% size  — mid-stream truncation (partial event data)

For each truncation, perf report is run and the exit code is checked:

- Exit code 0 (success) fails the test — a truncated file should
  never parse without error.

- Crash signals are detected portably via kill -l, which maps the
  signal number to a name on the running system.  This handles
  architectures where signal numbers differ (e.g. SIGBUS is 7 on
  x86/ARM but 10 on MIPS/SPARC).  Core-dump and fatal signals
  (KILL, ILL, ABRT, BUS, FPE, SEGV, TRAP, SYS) fail the test.

- Higher exit codes (200+) are perf's own negative-errno returns
  (e.g. -EINVAL = 234) and are expected.

This exercises the bounds checking, minimum-size validation, and error
propagation added by the preceding patches in this series.

Testing it:

  root@number:~# perf test truncat
   84: Test that perf report handles truncated perf.data gracefully (no crash, no segfault — clean error exit).: Ok
  root@number:~# perf test -vv truncat
   84: Test that perf report handles truncated perf.data gracefully (no crash, no segfault — clean error exit).:
  --- start ---
  test child forked, pid 62890
  ---- end(0) ----
   84: Test that perf report handles truncated perf.data gracefully (no crash, no segfault — clean error exit).: Ok
  root@number:~#

Changes in v2:
- Add SIGKILL to the list of fatal signals so OOM kills from
  resource exhaustion bugs are detected (Reported-by: sashiko-bot@kernel.org)

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Assisted-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
[ Fixed the SPDX on the line where 'perf test' expects the test description, reviewed by Ian Rogers ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>