libalias tests: Mark an unused variable with __unused.

It is not trivial to remove this set but unused variable since it
assigned to inside of a macro body, so just mark it with __unused
instead.

tests/libalias: Portrange

Test ranges of allowed ports for aliasing.
- Explicit default like ipfw(8) is doing
- Regular range
- Exhausting a very small range
- Recovery

Includes a fix of an utility macro, which was not used before.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31012

tests/libalias: Factor out common primitives

Rework the tests to check the correct layer in a single test.
Factor out tests for reuse in other modules.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30412

tests/libalias: Add missing copyright statements

tests/libalias: Reduce stress

Due to the new, external performance test utility, the regular test
suite can reduce the stress test to a fair amount of activity.

MFC after: 1 week

test/libalias: Tests for outgoing NAT

Testing LibAliasOut functionality. This concentrates the typical use
case of initiating data transfers from the inside. Provide a
exhaustive test for the data structure in order to check for
performance improvements.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30335