ZTS: pool_checkpoint improvements

The pool_checkpoint tests may incorrectly fail because several of
them invoke zdb for an imported pool. In this scenario it's not
unexpected for zdb to fail if the pool is modified. To resolve
this these zdb checks are now done after the pool has been exported.

Additionally, the default cleanup functions assumed the pool would
be imported when they were run. If this was not the case they're
exit early and fail to cleanup all of the test state causing
subsequent tests to fail. Add a check to only destroy the pool
when it is imported.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11832