selftests: Add duplicate config only for MD5 VRF tests

Commit referenced below added configuration in the default VRF that
duplicates a VRF to check MD5 passwords are properly used and fail
when expected. That config should not be added all the time as it
can cause tests to pass that should not (by matching on default VRF
setup when it should not). Move the duplicate setup to a function
that is only called for the MD5 tests and add a cleanup function
to remove it after the MD5 tests.

Fixes: 5cad8bce ("fcnal-test: Add TCP MD5 tests for VRF")
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>