MAC/do: Add consistency tests

Test that:
1. Concurrent changes to different parameters on the same jail are
independent/atomic.
2. Inheritance works.
3. Relaxing only parent jail rules does not l

MAC/do: Add consistency tests

Test that:
1. Concurrent changes to different parameters on the same jail are
independent/atomic.
2. Inheritance works.
3. Relaxing only parent jail rules does not leak to a subjail thanks to
sequential consistency.
4. Sysctl knobs and jail parameters stay consistent.

Some of these tests may be extended in the future with several layers of
jails (there is only a single subjail currently).

Reviewed by: bapt
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Pull Request: https://ron-dev.freebsd.org/FreeBSD/src/pulls/38

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