ping tests: Test IHL/quoted data/inner packet paths

Commit 46d7b45a267b3d78c5054b210ff7b6c55bfca42b introduced these code
paths. Test and document them.

• Add inner packet too short test
• Add inner IHL too short test
• Add quoted data too short test
• Add IHL too short test
• Add max inner packet IHL without payload test

Reviewed by: markj
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/863
Differential Revision: https://reviews.freebsd.org/D38528

(cherry picked from commit 20012a3a1a045bfe02bb64063cf0aba4d82471cb)