perf tests: Harden branch stack sampling test

perf tests: Harden branch stack sampling test

On continuous testing the perf script output can be empty, or nearly
empty, causing tr/grep to exit and due to "set -e" the test traps and
fails.

Add some empty file handling that sets the test to skip and make grep
and other text rewriting failures non-fatal by adding "|| true".

Committer testing:

  root@number:~# grep -m1 "model name" /proc/cpuinfo
  model name	: AMD Ryzen 9 9950X3D 16-Core Processor
  root@number:~# perf test "Check branch stack sampling"
  104: Check branch stack sampling                                     : Ok
  root@number:~#
  root@number:~# perf test -vvvvvvv "Check branch stack sampling"
  104: Check branch stack sampling:
  --- start ---
  test child forked, pid 396047
   142d22-142da0 l brstack_bench
  perf does have symbol 'brstack_bench'
  Testing user branch stack sampling
  Testing branch stack filtering permutation (any_call,CALL|IND_CALL|COND_CALL|SYSCALL|IRQ)
  Testing branch stack filtering permutation (call,CALL|SYSCALL)
  Testing branch stack filtering permutation (cond,COND)
  Testing branch stack filtering permutation (any_ret,RET|COND_RET|SYSRET|ERET)
  Testing branch stack filtering permutation (call,cond,CALL|SYSCALL|COND)
  Testing branch stack filtering permutation (any_call,cond,CALL|IND_CALL|COND_CALL|IRQ|SYSCALL|COND)
  Testing branch stack filtering permutation (cond,any_call,any_ret,COND|CALL|IND_CALL|COND_CALL|SYSCALL|IRQ|RET|COND_RET|SYSRET|ERET)
  ---- end(0) ----
  104: Check branch stack sampling                                     : Ok
  root@number:~#

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: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250318161639.34446-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>