exterror tests: harden the checks

Skip the message-content check on kernels that do not advertise the
exterr_strings feature, and pin the output format by clearing
EXTERROR_VERBOSE.

Reviewed by: kib
MFC after: 1 week
Assisted-by: Claude Code (Fable 5)
Differential Revision: https://reviews.freebsd.org/D58322

(cherry picked from commit 1092aba1260ce5efb0d718352aa22158605c7506)