ztest: Fix spa_open() ENOENT failures

ztest: Fix spa_open() ENOENT failures

The pool may not be imported when the previous pass is terminated.
In which case, spa_open() will return ENOENT to indicate the pool
is not currently imported.  Refactor to code slightly to handle
this case by importing the pool and then retrying the spa_open().

The ztest_import() function was moved before ztest_run() and the
import logic split in to a small internal helper function.  The
ztest_freeze() function was also moved but no changes were made.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #10407