Shorten arcstat_quiescence sleep time

With the latest L2ARC fixes, 2 seconds is too long to wait for
quiescence of arcstats like l2_size. Shorten this interval to avoid
having the persistent L2ARC tests in ZTS prematurely terminated.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14981