selftests: Extend fib tests to run with and without flags notifications

Run the test cases with both `fib_notify_on_flag_change` sysctls set to
'1', and then with both sysctls set to '0' to verify there are no
regressions in the test when notifications are added.

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