ZTS: fix wait_scrubbed()

Currently, wait_scrubbed() is the only function of its kind that
accepts a timeout, which is 10s by default. This timeout is pretty
short for a scrub and causes test failures if we run too long. This
patch removes the timeout, instead leaning on the global test suite
timeout to ensure the tests keep moving.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8210