net/packetdrill: fix build on armv7, FreeBSD 12
- make a symbol new in FreeBSD 13 optional
- mark a union as packed that will require unaligned access
- define TEST_TARGET
Approved by: portmgr (build fix blanket)
MFH: 2023Q4
(cherry picked from commit f6c3a4092779ca6fc4db3cf5bcb387fa3b268921)