bectl(8): Add some regression tests

These tests operate on a file-backed zpool that gets created in the kyua
temp dir. root and ZFS support are both required for these tests. Current
tests cover create, destroy, export/import, jail, list (kind of), mount,
rename, and jail.

List tests should later be extended to cover formatting and the different
list flags, but for now only covers basic "are create/destroy actually
reflected properly"

MFC after: 3 days