perf tests attr: Add optional term

Some of the stat events are quite rare to find on common machines (like
front end cycles).

Adding an 'optional' term to mark such events in attr tests. Event
marked as optional will not fail the test case if it's not found in
results.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20170703145030.12903-15-jolsa@kernel.org


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