ZTS: Improve cleanup in zpool tests

• Restore original kern.corefile value after the test.
• Don't leave behind a frozen pool.
• Clean up leftover vdev files.
• Make zpool_002_pos and zpool_003_pos consistent in their handling of

core files while here.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11694