$(( .. )) is a bash feature but the test's interpreter is !/bin/sh,
switch the code to use expr.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: bpf@vger.kernel.org
Link: http://lore.kernel.org/lkml/20210617184216.2075588-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Add a test to compare the output of perf-stat with and without option
--bpf-counters. If the difference is more than 10%, the test is considered
as failed.

Committer testing:

  # perf test bpf-counters
  86: perf stat --bpf-counters test                                   : Ok
  # perf test -v bpf-counters
  86: perf stat --bpf-counters test                                   :
  --- start ---
  test child forked, pid 2433339
  test child finished with 0
  ---- end ----
  perf stat --bpf-counters test: Ok
  #

Signed-off-by: Song Liu <songliubraving@fb.com>
Requested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/EC00E37D-8587-4662-8E30-7AD5F874FA84@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>