tc-testing: ife: fix wrong teardown command in test b7b8

fix failures in the 'teardown' stage of test b7b8, probably a leftover of
commit 7c5995b3 ("tc-testing: fixed copy-pasting error in ife tests")

Fixes: a56e6bcd ("tc-testing: updated ife test cases")
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>