ZTS: zed_start should not fail if zed is already running

zed_start may be called in places where zed is not
typically already running, but this is not a requirement
of the tests.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #9974