zloop: Add a max iterations option, use default run/pass times

It is useful to have control over the number of iterations of zloop so
we can easily produce "x core dumps found *in y iterations*" metrics.

Using random values for run/pass times doesn't improve coverage in a
meaningful way.

Randomizing run time could be seen as a compromise between running a
greater variety of shorter tests versus a smaller variety of longer
tests within a fixed time span. However, it is not desirable when
running a fixed number of iterations.

Pass time already incorporates randomness within ztest.

Either parameter can be passed to ztest explicitly if the defaults are
not satisfactory.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #12411