VN packet injection tests

[anrossi]

Describe the bug

In order to exercise cases where an attacker is on-path, or when the peer misconfigures version negotiation, there needs to be a way to inject version negotiation packets or alter them in transit.

This could be accomplished using the datapath test hooks to intercept a version negotiation packet and modify it, or to inject extra version negotiation packets.

Test cases specifically to be tested by this would be (not an exhaustive list):

• Ensure client doesn't respond to more than 1 VN packet (testing downgrade protection)
• Inject a VN packet when the server didn't send one (also testing downgrade protection)
• Putting a version in the VN packet the server didn't send.

Affected OS

• All