ZTS: testing for leaked key mappings in encrypted non-raw send

This test covers a bug fixed by commit ea74cde: performing an
incremental non-raw send from an encrypted filesystem followed by
exporting the pool. Before that commit, exporting the sending pool
in this scenario would trigger a panic:

VERIFY(avl_is_empty(&sk->sk_dsl_keys)) failed
PANIC at dsl_crypt.c:353:spa_keystore_fini()
Call Trace:
spl_dumpstack+0x29/0x2f [spl]
spl_panic+0xd1/0xe9 [spl]
spl_assert.constprop.0+0x1a/0x30 [zfs]
spa_keystore_fini+0xc2/0xf0 [zfs]
spa_deactivate+0x25f/0x610 [zfs]
spa_evict_all+0xf4/0x200 [zfs]
spa_fini+0x13/0x140 [zfs]
zfs_kmod_fini+0x72/0xc0 [zfs]
openzfs_fini_os+0x13/0x3a [zfs]
openzfs_fini+0x9/0x6b8 [zfs]

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #17366