ztest: Fix ASSERT in ztest_objset_destroy_cb()

The dsl_destroy_snapshot() call in ztest_objset_destroy_cb() may
encounter a runtime error when the pool is out of space. This is
similar to the error handling for the dsl_destroy_head() case,
but since dsl_destroy_snapshot() is implemented as a channel
program ECHRNG is returned instead of ENOSPC. ECHRNG may also
be returned instead of EBUSY if there is a hold on the snapshot.

Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13155