LU-12802 tests: speedup cleanup of racer

LU-12802 tests: speedup cleanup of racer

After racer test survives for a given time, it starts to cleanup.
And the parent racer.sh script waits the child racer/racer.sh
to exit. However sometimes, somehow, this stucks for a long time.
Sending a signal to remaining dd(or other) processes will wake up
the wait in parent racer.sh script immediately.

Lustre-change: https://review.whamcloud.com/35101

Test-Parameter: trivial testlist=racer
DDN-Bug-ID: DDN-256
Signed-off-by: Li Xi <lixi@ddn.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I2ff2784b76faa0532c39af29b1586a48f2b90a21
Reviewed-on: https://review.whamcloud.com/36289
Reviewed-by: Shilong Wang <wshilong@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>