tests: disable transmit checksum offloading for epair interfaces

When transmit checksum offloading is enabled, if_epair does not
compute checksums, it just marks packets that this computation still
needs to be done. However, some test cases verify the checksum.
Therefore disable this for IPv4 and IPv6.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52379