perf tests attr: Fix group stat tests

perf tests attr: Fix group stat tests

We started to use group read whenever it's possible:

  82bf311e15d2 perf stat: Use group read for event groups

That breaks some of attr tests, this change adds the new possible
read_format value.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
LPU-Reference: 20170928160633.GA26973@krava
Link: http://lkml.kernel.org/n/tip-1ko2zc4nph93d8lfwjyk9ivz@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>