network: several fixlets for NDisc (#42218)

network: several fixlets for NDisc (#42218)

Unfortunately, previously the path to test-ndisc-send has been wrong, so
some test cases have not been checked in our mkosi CIs. And two test
cases have been broken.

The test case `test_ndisc_redirect` was not updated when the logic in
networkd was changed by 9142bd5a8e9ed94ecbb1e335305e24760b90ad2a. The
change itself should be OK. So, the test case is updated.

The test case `test_ndisc_mtu` was broken when the commit
32417c172383847ec78b672c537594e3efe8f0e0 is merged. The commit is not
correct, as we cannot set IPv6 MTU larger than interface MTU. So, the
offending commit is reverted.