perf test: Add expand cgroup event test

It'll expand given events for cgroups A, B and C.

  $ perf test -v expansion
  69: Event expansion for cgroups                      :
  --- start ---
  test child forked, pid 983140
  metric expr 1 / IPC for CPI
  metric expr instructions / cycles for IPC
  found event instructions
  found event cycles
  adding {instructions,cycles}:W
  copying metric event for cgroup 'A': instructions (idx=0)
  copying metric event for cgroup 'B': instructions (idx=0)
  copying metric event for cgroup 'C': instructions (idx=0)
  test child finished with 0
  ---- end ----
  Event expansion for cgroups: Ok

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200924124455.336326-6-namhyung@kernel.org


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