test: Diagnose check-networking failures related to mtu
test: Diagnose check-networking failures related to mtu

Sometimes the 'mtu 1400' change doesn't seem to take, and tests
fail. Lets try logging the output of 'ip link show' to the journal
so that we can see what happens next time.

Closes #5180
Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

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