ZTS: adjust zpool_import_012_pos timeout

When running in the CI the zpool_import_012_pos test case occasionally
takes longer than the maximum 600 seconds. When this happens the test
case is considered to have failed but always completes a few minutes
latter. Since the logs suggest nothing has actually failed this commit
increases timeout and removes the exception.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11286