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

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

In this test, a large ring buffer is required so all events can feed
into, so overwrite or not is meaningless.

Change to !overwrite so following commits can remove this argument.

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/20171113013809.212417-5-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>