fix performance regression in test/bench/test.sh

ktap overhead increased when using task-bound tracing,
the reason is there have a lot of perf-event schedule
in and schedule out when using task-bound tracing.

So change it back to system tracing.

Before:
run time 30 seconds 200248 worker burns per second

After:
run time 30 seconds 250742 worker burns per second

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>