tests: iptables-test: Fix for duplicate supposed-to-fail errors

tests: iptables-test: Fix for duplicate supposed-to-fail errors

Unexpected results for lines which are supposed to fail are reported
twice: Once when fast mode runs them individually to clear the path
before batch-handling all others, a second time when non-fast mode takes
over after fast mode had failed and runs all tests individually again.

Sort this nuisance by running these tests silently in fast mode, knowing
that they will run again if failing anyway.

Fixes: 0e80cfea3762b ("tests: iptables-test: Implement fast test mode")
Signed-off-by: Phil Sutter <phil@nwl.cc>