Test that we properly ignore the Reserved fields of the IKEv2 Proposal ID payload

We use an impair to set it to a bogus value and check that the connection still establishes

The reason this is tested differently, is that these reserved fields are part of the input to
the PRF for the AUTH payload. So if you ignore these fields in in_struct() because they should
be zero anyway, the two endpoints will calculate a different hash for the AUTH payload and the
IKE SA will fail to establish. This would happen on libreswan < 3.33 if the remote peer had
such a bug of sending a bad ID reserved field.
