Skip import activity test in more zdb code paths

Since zdb opens the pools read-only, it cannot damage the pool in the
event the pool is already imported either on the same host or on
another one.

If the pool vdev structure is changing while zdb is importing the
pool, it may cause zdb to crash. However this is unlikely, and in any
case it's a user space process and can simply be run again.

For this reason, zdb should disable the multihost activity test on
import that is normally run.

This commit fixes a few zdb code paths where that had been overlooked.
It also adds tests to ensure that several common use cases handle this
properly in the future.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Gu Zheng <guzheng2331314@163.com>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #7797
Closes #7801