perf tests: Set evlist of test__task_exit() to !overwrite

perf tests: Set evlist of test__task_exit() to !overwrite

Changing ringbuffer to !overwrite in this task is harmless because
this test uses a very low frequency (1) and using a very simple program
(true). There should have only 3 events in the whole test.  Overwriting
is impossible to happen.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20171113013809.212417-6-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>