perf tests: Skip branch stack sampling test if brstack_bench symbol is missing

perf tests: Skip branch stack sampling test if brstack_bench symbol is missing

The test "Check branch stack sampling" depends on finding symbol
brstack_bench (and several others) in perf, and fails if perf has been
stripped and no debug object is available. In that case, skip the test
instead.

Example:

 Before:

  $ strip tools/perf/perf
  $ tools/perf/perf buildid-cache -p `realpath tools/perf/perf`
  $ tools/perf/perf test -v 'branch stack sampling'
  112: Check branch stack sampling                                     :
  --- start ---
  test child forked, pid 123741
  Testing user branch stack sampling
  + grep -E -m1 ^brstack_bench\+[^ ]*/brstack_foo\+[^ ]*/IND_CALL/.*$ /tmp/__perf_test.program.5Dz1U/perf.script
  + cleanup
  + rm -rf /tmp/__perf_test.program.5Dz1U
  test child finished with -1
  ---- end ----
  Check branch stack sampling: FAILED!

 After:

  $ tools/perf/perf test -v 'branch stack sampling'
  112: Check branch stack sampling                                     :
  --- start ---
  test child forked, pid 125157
  perf does not have symbol 'brstack_bench'
  perf is missing symbols - skipping test
  test child finished with -2
  ---- end ----
  Check branch stack sampling: Skip

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: German Gomez <german.gomez@arm.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20231123075848.9652-7-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>