selftests: netdevsim: Add fib_notifications test

Add test to check fib notifications behavior.

The test checks route addition, route deletion and route replacement for
both IPv4 and IPv6.

When fib_notify_on_flag_change=0, expect single notification for route
addition/deletion/replacement.

When fib_notify_on_flag_change=1, expect:
- two notification for route addition/replacement, first without RTM_F_TRAP
  and second with RTM_F_TRAP.
- single notification for route deletion.

$ ./fib_notifications.sh
TEST: IPv4 route addition                                           [ OK ]
TEST: IPv4 route deletion                                           [ OK ]
TEST: IPv4 route replacement                                        [ OK ]
TEST: IPv6 route addition                                           [ OK ]
TEST: IPv6 route deletion                                           [ OK ]
TEST: IPv6 route replacement                                        [ OK ]

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>