remove sanity check in replacement test

remove sanity check in replacement test

In replacement test, it spawns a process to truncate a file background
and make sure that the process exists 1 second later. However, the
process may have finished its work and exited therefore it has the
chance to report a false alarm.

This patch just removed those sanity check.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4516