Rewrite pfctl_test in C to reduce testsuite run time

Rewrite pfctl_test in C to reduce testsuite run time

The new C test takes 25 seconds on QEMU-RISC-V, wheras the shell version
takes 332 seconds.

Even with the latest optimizations to atf-sh this test still takes a few
seconds to startup in QEMU. Re-writing it in C reduces the runtime for a
single test from about 2-3 seconds to less than .5 seconds. Since there
are ~80 tests, this adds up to about 3-4 minutes.
This may not seem like a big speedup, but before the recent optimizations
to avoid atf_get_srcdir, each test took almost 100 seconds on QEMU RISC-V
instead of 3. This also significantly reduces the time it takes to list
the available test cases, which speeds up running the tests via kyua:

```
root@qemu-riscv64-alex:~ # /usr/bin/time kyua test -k /usr/tests/sbin/pfctl/Kyuafile pfctl_test_old
...
158/158 passed (0 failed)
      332.08 real        42.58 user       286.17 sys
root@qemu-riscv64-alex:~ # /usr/bin/time kyua test -k /usr/tests/sbin/pfctl/Kyuafile pfctl_test
158/158 passed (0 failed)
       24.96 real         9.75 user        14.26 sys

root@qemu-riscv64-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test pf1001
pfctl_test: WARNING: Running test cases outside of kyua(1) is unsupported
pfctl_test: WARNING: No isolation nor timeout control is being applied; you may get unexpected failures; see atf-test-case(4)
Running pfctl -o none -nvf /usr/tests/sbin/pfctl/./files/pf1001.in
---
binat on em0 inet6 from fc00::/64 to any -> fc00:0:0:1::/64
binat on em0 inet6 from any to fc00:0:0:1::/64 -> fc00::/64
---
passed
        0.17 real         0.06 user         0.08 sys
root@qemu-riscv64-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test_old pf1001
pfctl_test_old: WARNING: Running test cases outside of kyua(1) is unsupported
pfctl_test_old: WARNING: No isolation nor timeout control is being applied; you may get unexpected failures; see atf-test-case(4)
Id  Refs Name
141    1 pf
Executing command [ pfctl -o none -nvf - ]
passed
        1.73 real         0.25 user         1.41 sys
root@qemu-riscv64-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test_old -l > /dev/null
       24.36 real         2.26 user        21.86 sys
root@qemu-riscv64-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test -l > /dev/null
        0.04 real         0.02 user         0.01 sys
```

The speedups are even more noticeable on CHERI-RISC-V (since QEMU runs
slower when emulating CHERI instructions):
```
root@qemu-cheri-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test_new -l > /dev/null
        0.51 real         0.49 user         0.00 sys
root@qemu-cheri-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test -l > /dev/null
       34.20 real        32.69 user         0.16 sys
root@qemu-cheri-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test pf1001
pfctl_test: WARNING: Running test cases outside of kyua(1) is unsupported
pfctl_test: WARNING: No isolation nor timeout control is being applied; you may get unexpected failures; see atf-test-case(4)
Id  Refs Name
147    1 pf
Executing command [ pfctl -o none -nvf - ]
passed
        5.74 real         5.41 user         0.03 sys
root@qemu-cheri-alex:/usr/tests/sbin/pfctl # /usr/bin/time ./pfctl_test_new pf1001
pfctl_test_new: WARNING: Running test cases outside of kyua(1) is unsupported
pfctl_test_new: WARNING: No isolation nor timeout control is being applied; you may get unexpected failures; see atf-test-case(4)
Running pfctl -o none -nvf /usr/tests/sbin/pfctl/./files/pf1001.in
---
binat on em0 inet6 from fc00::/64 to any -> fc00:0:0:1::/64
binat on em0 inet6 from any to fc00:0:0:1::/64 -> fc00::/64
---
passed
        0.68 real         0.66 user         0.00 sys
root@qemu-cheri-alex:/usr/tests/sbin/pfctl #
```

Reviewed By:	kp
Differential Revision: https://reviews.freebsd.org/D26779