selftests: fib_nexthops: List each test case in a different line

The lines with the IPv4 and IPv6 test cases are already very long and
more test cases will be added in subsequent patches.

List each test case in a different line to make it easier to extend the
test with more test cases.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>