Add tests for the new sock/skb operators

This commit adds tests to confirm that the sock/skb operators are
working correctly. Limited the new tests to TCP sock only, because skb
works in exactly the same way and adding tests for UDP and/or skb would
only duplicate tests for little benefit.

Also, stop checking protocol on sock tests as this is unreliable.

Signed-off-by: Kevin Sheldrake <kevin.sheldrake@isovalent.com>